MCAT General Chemistry Review - Steven A. Leduc 2015


General Chemistry Strategy for the MCAT

2.1 SCIENCE SECTIONS OVERVIEW

There are three science sections on the MCAT:

• Chemical and Physical Foundations of Biological Systems

• Biological and Biochemical Foundations of Living Systems

• Psychological, Social, and Biological Foundations of Behavior

The Chemical and Physical Foundations of Biological Systems section (Chem/Phys) is the third section on the test. It includes questions from General Chemistry (about 35%), Physics (about 25%), Organic Chemistry (about 15%), and Biochemistry (about 25%). Further, the questions often test chemical and physical concepts within a biological setting, for example, pressure and fluid flow in blood vessels. A solid grasp of math fundamentals is required (arithmetic, algebra, graphs, trigonometry, vectors, proportions, and logarithms), however there are no calculus-based questions.

The Biological and Biochemical Foundations of Living Systems section (Bio/Biochem) is the first section on the test. Approximately 65% of the questions in this section come from biology, approximately 25% come from biochemistry, and approximately 10% come from Organic and General Chemistry. Math calculations are generally not required on this section of the test, however a basic understanding of statistics as used in biological research is helpful.

The Psychological, Social, and Biological Foundations of Behavior section (Psych/Soc) is the fourth and final section on the test. About 60% of the questions will be drawn from Psychology, about 30% from Sociology, and about 10% from Biology. As with the Bio/Biochem section, calculations are generally not required, however a basic understanding of statistics as used in research is helpful.

Most of the questions in the science sections (about 75%) are passage-based, and each section will likely have about nine or ten passages. Passages consist of a few paragraphs of information and include equations, reactions, graphs, figures, tables, experiments, and data. Five to seven questions will be associated with each passage.

The remaining 25% of the questions in each science section are freestanding questions (FSQs). These questions appear in groups interspersed between the passages. Each group contains four to five questions.

95 minutes are allotted to each of the science sections. This breaks down to approximately one minute and 25 seconds per question.

2.2 GENERAL SCIENCE PASSAGE TYPES

The passages in the science sections fall into one of three main categories: Information and/or Situation Presentation, Experiment/Research Presentation, or Persuasive Reasoning.

Information and/or Situation Presentation

These passages either present straightforward scientific information or they describe a particular event or occurrence. Generally, questions associated with these passages test basic science facts or ask you to predict outcomes given new variables or new information. Here is an example of an Information/Situation Presentation passage:

Figure 1 shows a portion of the inner mechanism of a typical home smoke detector. It consists of a pair of capacitor plates which are charged by a 9-volt battery (not shown). The capacitor plates (electrodes) are connected to a sensor device, D; the resistor R denotes the internal resistance of the sensor. Normally, air acts as an insulator and no current would flow in the circuit shown. However, inside the smoke detector is a small sample of an artificially produced radioactive element, americium-241, which decays primarily by emitting alpha particles, with a half-life of approximately 430 years. The daughter nucleus of the decay has a half-life in excess of two million years and therefore poses virtually no biohazard.

Image

Figure 1 Smoke detector mechanism

The decay products (alpha particles and gamma rays) from the 241Am sample ionize air molecules between the plates and thus provide a conducting pathway which allows current to flow in the circuit shown in Figure 1. A steady-state current is quickly established and remains as long as the battery continues to maintain a 9-volt potential difference between its terminals. However, if smoke particles enter the space between the capacitor plates and thereby interrupt the flow, the current is reduced, and the sensor responds to this change by triggering the alarm. (Furthermore, as the battery starts to “die out,” the resulting drop in current is also detected to alert the homeowner to replace the battery.)

C = ε0Image

Equation 1

where ε0 is the universal permittivity constant, equal to 8.85 × 10−12 C2/(N·m2). Since the area A of each capacitor plate in the smoke detector is 20 cm2 and the plates are separated by a distance d of 5 mm, the capacitance is 3.5 × 10−12 F = 3.5 pF.

Experiment/Research Presentation

These passages present the details of experiments and research procedures. They often include data tables and graphs. Generally, questions associated with these passages ask you to interpret data, draw conclusions, and make inferences. Here is an example of an Experiment/Research Presentation passage:

The development of sexual characteristics depends upon various factors, the most important of which are hormonal control, environmental stimuli, and the genetic makeup of the individual. The hormones that contribute to the development include the steroid hormones estrogen, progesterone, and testosterone, as well as the pituitary hormones FSH (follicle-stimulating hormone) and LH (luteinizing hormone).

To study the mechanism by which estrogen exerts its effects, a researcher performed the following experiments using cell culture assays.

Experiment 1:

Human embryonic placental mesenchyme (HEPM) cells were grown for 48 hours in Dulbecco’s Modified Eagle Medium (DMEM), with media change every 12 hours. Upon confluent growth, cells were exposed to a 10 mg per mL solution of green fluorescent-labeled estrogen for 1 hour. Cells were rinsed with DMEM and observed under confocal fluorescent microscopy.

Experiment 2:

HEPM cells were grown to confluence as in Experiment 1. Cells were exposed to Pesticide A for 1 hour, followed by the 10 mg/mL solution of labeled estrogen, rinsed as in Experiment 1, and observed under confocal fluorescent microscopy.

Experiment 3:

Experiment 1 was repeated with Chinese Hamster Ovary (CHO) cells instead of HEPM cells.

Experiment 4:

CHO cells injected with cytoplasmic extracts of HEPM cells were grown to confluence, exposed to the 10 mg/mL solution of labeled estrogen for 1 hour, and observed under confocal fluorescent microscopy.

The results of these experiments are given in Table 1.

Table 1 Detection of Estrogen (+ indicates presence of Estrogen)

Image

After observing the cells in each experiment, the researcher bathed the cells in a solution containing 10 mg per mL of a red fluorescent probe that binds specifically to the estrogen receptor only when its active site is occupied. After 1 hour, the cells were rinsed with DMEM and observed under confocal fluorescent microscopy. The results are presented in Table 2.

The researcher also repeated Experiment 2 using Pesticide B, an estrogen analog, instead of Pesticide A. Results from other researchers had shown that Pesticide B binds to the active site of the cytosolic estrogen receptor (with an affinity 10,000 times greater than that of estrogen) and causes increased transcription of mRNA.

Table 2 Observed Fluorescence and Estrogen Effects (G = green, R = red)

Image

Based on these results, the researcher determined that estrogen had no effect when not bound to a cytosolic, estrogen-specific receptor.

Persuasive Reasoning

These passages typically present a scientific phenomenon along with a hypothesis that explains the phenomenon, and may include counter-arguments as well. Questions associated with these passages ask you to evaluate the hypothesis or arguments. Persuasive Reasoning passages in the science sections of the MCAT tend to be less common than Information Presentation or Experiment-based passages. Here is an example of a Persuasive Reasoning passage:

Two theoretical chemists attempted to explain the observed trends of acidity by applying two interpretations of molecular orbital theory. Consider the pKa values of some common acids listed along the conjugate base:

Image

Recall that acids with a pKa < 0 are called strong acids, and those with a pKa > 0 are called weak acids. The arguments of the chemists are given below.

Chemist #1:

“The acidity of a compound is proportional to the polarization of the H—X bond, where X is some nonmetal element. Complex acids, such as H2SO4, HClO4, and HNO3 are strong acids because the H—O bonding electrons are strongly drawn towards the oxygen. It is generally true that a covalent bond weakens as its polarization increases. Therefore, one can conclude that the strength of an acid is proportional to the number of electronegative atoms in that acid.”

Chemist #2:

“The acidity of a compound is proportional to the number of stable resonance structures of that acid’s conjugate base. H2SO4, HClO4, and HNO3 are all strong acids because their respective conjugate bases exhibit a high degree of resonance stabilization.”

MAPPING OUT A PASSAGE

“Mapping a passage” refers to the combination of on-screen highlighting and scratch paper notes that you take while working through a passage. Typically, good things to highlight include the overall topic of a paragraph, familiar terms, unusual terms, numerical values, hypothesis, and results. Scratch paper notes can be used to summarize the paragraphs and to jot down important facts and connections that are made when reading the passage. Remember that highlighting disappears once you leave the passage, so a good set of scratch paper notes can be extremely useful if you have to return to the passage. More details on passage mapping will be presented in Section 2.5.

2.3 GENERAL SCIENCE QUESTION TYPES

Question in the science sections are generally one of three main types: Memory, Explicit, or Implicit.

Memory Questions

These questions can be answered directly from prior knowledge, with no need to reference the passage or question text. Memory questions represent approximately 25 percent of the science questions on the MCAT. Usually, Memory questions are found as FSQs, but they can also be tucked into a passage. Here’s an example of a Memory question:

Which of the following acetylating conditions will convert diethylamine into an amide at the fastest rate?

A) Acetic acid / HCl

B) Acetic anhydride

C) Acetyl chloride

D) Ethyl acetate

Explicit Questions

Explicit questions can be answered primarily with information from the passage, along with prior knowledge. They may require data retrieval, graph analysis, or making a simple connection. Explicit questions make up approximately 35—40 percent of the science questions on the MCAT; here’s an example (taken from the Information/Situation Presentation passage above):

The sensor device D shown in Figure 1 performs its function by acting as:

A) an ohmmeter.

B) a voltmeter.

C) a potentiometer.

D) an ammeter.

Implicit Questions

These questions require you to take information from the passage, combine it with your prior knowledge, apply it to a new situation, and come to some logical conclusion. They typically require more complex connections than do Explicit questions, and may also require data retrieval, graph analysis, etc. Implicit questions usually require a solid understanding of the passage information. They make up approximately 35—40 percent of the science questions on the MCAT; here’s an example (taken from the Experiment/Research Presentation passage above):

If Experiment 2 were repeated, but this time exposing the cells first to Pesticide A and then to Pesticide B before exposing them to the green fluorescent-labeled estrogen and the red fluorescent probe, which of the following statements will most likely be true?

A) Pesticide A and Pesticide B bind to the same site on the estrogen receptor.

B) Estrogen effects would be observed.

C) Only green fluorescence would be observed.

D) Both green and red fluorescence would be observed.

2.4 GENERAL CHEMISTRY ON THE MCAT

Although general chemistry is sometimes remembered as a daunting topic from college, the MCAT does not test the fine details of general chemistry. Rather, the focus of this section is on having a strong knowledge of chemistry fundamentals, and manipulating that knowledge to adapt to different scenarios presented in passages and questions. The passages often contain information that recapitulates basic chemistry knowledge, and may present additional information that builds on fundamental concepts.

The majority of the G-Chem questions will not be based on rote memory, but will require you to retrieve information from the passage and use some deductive reasoning skills. Thus, in order to succeed in this section, you not only need solid knowledge of fundamental principles of chemistry, but also strong critical reasoning and reading comprehension skills. These three components may be stressed differently depending on the passage type.

MCAT 2015 is likely to have around 9-10 passages and about 17 freestanding questions (FSQs) in each of the science sections. General Chemistry will make up about a third of the questions in the Chemical and Physical Foundations of Biological Systems section. The remaining questions will be on Physics (25%), Organic Chemistry (15%), and Biochemistry (25%). In addition, about 5% of the questions on the Biological and Biochemical Foundations of Living Systems section will be General Chemistry.

2.5 PASSAGE TYPES AS THEY APPLY TO GENERAL CHEMISTRY

Information/Situation Presentation: G-Chem

These passages assume knowledge of basic scientific concepts, and also present new information that builds on these basic concepts. The new information may be presented in a way that is very similar to how it would appear in a textbook or other scientific reference. The questions may be about basic scientific facts that you already know, but often the passage will present topics or subtopics with which you are unfamiliar. Information/Situation Presentation passages can be intimidating, as they often explore topics in a greater level of detail than the scope of your MCAT preparation. However, keep in mind that the whole point of these types of passages is to force you to use critical reasoning and apply your basic scientific knowledge to new topics. It is not to see how much advanced scientific coursework you have memorized. Therefore, it is important when you see a passage on, say, molecular orbital theory, that you don’t think to yourself, “Oh no!! I forgot to study molecular orbital theory!!!” Rather, look at the information in the passage, and consider how your knowledge about more basic chemical concepts, such as electron configurations, can be applied in order to answer the questions. The new information in the passage can supplement your basic knowledge.

This type of passage may also present information in the context of a specific situation, such as the results of a research study or an experiment. In this case, the questions may ask you to distinguish between data that supports or refutes the result being presented. In some passages, an apparently contradictory or erroneous result is presented and questions may ask what mistakes could have been made over the course of the experiment to cause such a result. Thus, these passages require to you think critically about the importance of each chemical and physical element of an experiment. Note however, that they do not present the steps of an experiment in great detail; that style is reserved for Experiment/Research Presentation passages.

Experiment/Research Presentation: G-Chem

These passages present an experimental set up in great detail; they describe the rationale behind an experiment, how it is set up and executed, and its results. In these passages you are often asked to analyze data given in the form of charts and graphs. In addition, questions may ask you how the results of the experiment would differ if a certain variable were changed; this requires you to think critically about the role of each element of the experiment. In this passage type, be careful not to gloss over important experimental details as you retrieve information from the passage. Be aware that details such as units can make the difference between answering a question correctly or incorrectly, and be vigilant about these experimental details as you work through the questions and look back to the passage.

Persuasive Argument: G-Chem

In a Persuasive Argument passage, two perspectives on a problem are presented. It may be different researchers putting forth two different methodologies for conducting an experiment, or two different explanations for an experimental result or phenomenon.

The questions may ask how the authors came to develop different perspectives, or ask you to evaluate the credibility of each of their arguments. Persuasive Argument passages are the least common passage type in G-Chem.

READING A GENERAL CHEMISTRY PASSAGE

Reading a G-Chem passage is not like reading a scientific paper or a textbook. That is, you are not reading thoroughly and trying to understand the relevance of each sentence, as the passage will likely contain details beyond the scope of the questions. In fact, many of the questions can be answered without using any information from the passage.

Instead, your goal is to take no more than 30 to 60 seconds and skim the passage in order to determine the general topic area being tested and create a brief passage map before moving on to the questions. To do this as efficiently as possible, focus on the first sentence of each paragraph and any bolded or italicized words. In addition, chemical equations and figures may provide insight as to the general topic of the passage. For example, if you see a titration curve, it is likely that the passage will test acid-base chemistry.

G-Chem passages often include complex graphs and data tables. Avoid the temptation to analyze this data on your first pass through the passage. Rather, wait until you find a question that requires the use of the data in the graph or table, then analyze the data in the context of that question. This approach is more efficient and productive than trying to preemptively interpret data.

The bottom line: You can always go back and reread more details from the passage. Furthermore, not all of the details from the passage are necessary to answer the questions. Therefore, it is a waste of your time to read and attempt to thoroughly understand the passage the first time you read it.

MAPPING A G-CHEM PASSAGE

As you skim through a G-Chem passage to get a feel for the type of questions that might follow, take note of the general location of information within the passage. The highlighter is a useful way to visually note a few key words that relate to the general topic of the passage or some unusual or new term that is introduced. Use the highlighter sparingly, and keep in mind that any highlighting you do will not persist as you move from passage to passage. If you want to make more permanent notes, use the scratch paper. An example of a highlighted passage is shown below. This is an Information Presentation passage:

The batteries that start an automobile or power flashlights are devices that convert chemical energy into electrical energy. These devices use spontaneous oxidation-reduction reactions (called half-reactions) that take place at the electrodes to create an electric current. The strength of the battery, or electromotive force, is determined by the difference in electric potential between the half cells, expressed in volts. This voltage depends on which reactions occur at the anode and the cathode, the concentrations of the solutions in the cells, and the temperature. The cell voltage, E, at a temperature of 25°C and nonstandard conditions, can be calculated from the Nernst equation, where E° is the standard potential, n denotes the number of electrons transferred in the balanced half reaction, and Q is the reaction quotient.

E = E° − Image log10 Q

Equation 1

The lead storage battery used in automobiles is composed of six identical cells joined in series. The anode is solid lead, the cathode is lead dioxide, and the electrodes are immersed in a solution of sulfuric acid. As each cell discharges during normal operation, the sulfate ion is consumed as it is deposited in the form of lead sulfate on both electrodes, as shown in Reaction 1:

Reaction 1:

Image

Each cell produces 2 V, for a total of 12 V for the typical car battery. Unlike many batteries, however, the lead storage battery can be recharged by applying an external voltage. Because the redox reaction in the battery consumes sulfate ions, the degree of discharge of the battery can be checked by measuring the density of the battery fluid with a hydrometer. The fluid density in a fully charged battery is 1.2 g/cm3.

Table 1 Standard Reduction Potentials at T = 25°C

Half-reaction

E° (V)

F2(g) + 2e → 2F(aq)

+2.87

Cl2(g) + 2e → 2Cl(aq)

+1.36

Cu+(aq) + e → Cu(s)

+0.52

Cu2+(aq) + 2e → Cu(s)

+0.34

Zn2+(aq) +2e → Zn(s)

—0.76

Al3+(aq) + 3e → Al(s)

—1.66

Li+(aq) + e → Li(s)

—3.05

Note that only a few words are highlighted. In the first paragraph, “batteries” and “spontaneous oxidation-reduction” relate to the general topic of the passage, and serve as a reminder that batteries contain a spontaneous redox reaction. The second paragraph identifies the two electrodes in the battery and, in the last paragraph, the voltage of a car battery is highlighted. Since this is a specific and unusual piece of information, it is likely to come up in a question.

Rather than highlighting large portions of the passage as you skim it, use your scratch paper to create a simple passage map to help organize where different types of information are in the passage. Scratch paper is only useful if it is kept organized! Make sure that your notes for each passage are clearly delineated and marked with the passage number on your scratch paper. This will allow you to easily read your notes when you come back to review a marked question. Resist the temptation to write in the first available blank space, as this makes it much more difficult to refer back to your work.

As you skim the passage, note the subject of each paragraph and any key words or values. A well-constructed passage map makes it easier and more efficient to go back and retrieve specific information as you work through the questions. Here is an example of a passage map for the passage shown above:

P1 — Batteries, general information, background

P2 — Automobile batteries, more specific information about them

P3 — Recharging car battery, Reduction Potentials in Table 1

As you can see, your passage map does not need to be particularly detailed, nor should it be, as reading and mapping the passage should only take a minute of your time. However, this does provide a valuable framework for efficiently locating information within the passage.

Let’s look at another passage and how to map it. This is an Experiment/Research Presentation passage from The Princeton Review’s free online demo MCAT:

Two cube-shaped compartments, X and Y, each with a volume of one cubic meter, were used in several experiments to study the properties of gases. Compartment X was fitted with a piston of negligible mass which fit snugly against the walls of the container. The compartments were connected by a pinhole which could be opened or closed at will (see Figure 1). The pressure and temperature could be measured in either compartment. At the start of each experiment, Compartment X contained equal molar quantities of four gases (helium, oxygen, nitrogen, and carbon dioxide), the temperature in Compartment X was 25°C and the pressure was 1 atm. Initially, Compartment Y was evacuated. The behavior of all the gases can be assumed to be ideal. (Note: 1 atm ≈ 105 Pa.)

Image

Figure 1 Experimental apparatus

Experiment 1:

With the pinhole closed, the temperature of the gases in Compartment X was gradually increased to 50°C, and the pressure of the gas inside the compartment was measured.

Experiment 2:

With the pinhole closed, the piston was gradually lowered into Compartment X until it had dropped a distance of 0.5 m. The pressure of the gas in the container was then measured.

Experiment 3:

The pinhole was opened, and the pressure change in each compartment was measured until equilibrium was reached.

Here, the highlighter tool can be used to emphasize that this passage is about the behavior of gases. Any time a passage is about gases, it’s useful to know if the gas behaves in a real or ideal manner; therefore the phrase “assumed to be ideal” is also highlighted. In experimental passages, if important details jump out at you on your initial skim of the passage, it’s useful to highlight them. For example, Figure 1 makes it fairly obvious that compartment X contains four gases, while compartment Y is a vacuum with no gas, however the “equal molar quantities” of the four gases in compartment X is a useful detail to highlight. Here’s how you might map this passage on your scratch paper:

P1 — Experimental setup

E1 — Temp change

E2 — Pressure change

E3 — Pressure change, equilibrium

As was true of our last passage map, the main purpose is to create an outline so that it will be easier to retrieve necessary information as you work through the questions. Since this is an Experiment Presentation passage, the map points out the location of the main experimental details. Note that on the first pass, it is not important to note the specific details of each individual experiment on your scratch paper, though quickly highlighting new experimental conditions, like temperature, etc. can be helpful. If possible, however, it may be helpful to note the general variable being changed.

Let’s look at one more example of passage mapping. This passage is a Persuasive Argument Passage:

Two theoretical chemists attempted to explain the observed trends of acidity by applying two interpretations of molecular orbital theory. Consider the pKa values of some common acids listed along with the conjugate base of each acid:

Image

Recall that acids with a pKa < 0 are called strong acids, and those with a pKa > 0 are called weak acids. The arguments of the chemists are given below.

Chemist #1:

“The acidity of a compound is proportional to the polarization of the H—X bond, where X is some nonmetal element. Complex acids, such as H2SO4, HClO4, and HNO3 are strong acids because the H—O bonding electrons are strongly drawn towards the oxygen. It is generally true that a covalent bond weakens as its polarization increases. Therefore, one can conclude that the strength of an acid is proportional to the number of electronegative atoms in that acid.”

Chemist #2:

“The acidity of a compound is proportional to the number of stable resonance structures of that acid’s conjugate base. H2SO4, HClO4, and HNO3 are all strong acids because their respective conjugate bases exhibit a high degree of resonance stabilization.”

For a Persuasive Argument passage, the goal of passage mapping and highlighting is to identify the issue being addressed, and the main points of each of the opposing lines of reasoning. This can be accomplished using the highlighter tool to emphasize that the passage is about “trends of acidity”, and that Chemist #1 attributes the behavior of acids to “polarization of the H—X bond,” while Chemist #2 focuses on “stable resonance structures.”

In this case, a passage map would be very similar to the results achieved by highlighting. However, keep in mind that while highlighting does not persist as you move from passage to passage, a passage map can permanently be referred to on well-organized scratch paper. Also, the very act of writing things down helps clarify it in your head:

P1/Main issue: Trends of acidity, interpretation of molecular orbital theory

Chemist #1: aciditypolarization of H—X bond

Chemist #2: acidity# of stable resonance structures for conjugate base.

As you can see from the examples above, effective passage-mapping requires a combination of highlighting and jotting down notes in an organized fashion on your scratch paper. The best way to improve your passage mapping, and to determine which combination of these skills works best for you, is to practice, practice, practice.

2.6 TACKLING THE QUESTIONS

In general, G-Chem questions require a combination of basic knowledge, passage retrieval, and critical reasoning. The more difficult G-Chem questions tend to weigh the last two skills more heavily. Therefore, if you have a sound basis in the fundamental principles of General Chemistry, it is safe to assume that a tough question will be best addressed by looking back to the passage for information that is either explicitly stated or implied.

In the section on passage mapping, we reviewed an Information/Situation Presentation passage on batteries and redox reactions. We will draw on questions from this passage in order to illustrate the different question types.

G-Chem Memory Questions

These questions test background knowledge and require you to recall a specific definition or relationship. Memory questions are often freestanding questions, either on their own or within a set of questions that accompany a passage. In the latter instance, the information they require is not given in the passage. For example, a question from the car battery passage shown above asked:

If the reaction in a concentration cell is spontaneous in the reverse direction, then:

A) Q < K, ΔG for the forward reaction is negative, and the cell voltage is positive.

B) Q < K, ΔG for the forward reaction is positive, and the cell voltage is negative.

C) Q > K, ΔG for the forward reaction is negative, and the cell voltage is positive.

D) Q > K, ΔG for the forward reaction is positive, and the cell voltage is negative.

In order to answer this question correctly, you need to know the connection between ∆G and spontaneity. A spontaneous reaction has a negative ∆G, and a nonspontaneous reaction has a positive ∆G. Since the reaction is spontaneous in the reverse direction, it must be nonspontaneous in the forward direction. Therefore, the ∆G of the forward reaction is positive, eliminating choices A and C. Alternatively, you could know that cell voltage applies to the forward direction, and that a nonspontaneous cell has a negative voltage, also eliminating choices A and C.

To distinguish between choices B and D, you must have a fundamental understanding of equilibrium and Le Châtelier’s Principle. The reaction quotient, Q, always approaches the equilibrium constant, K, and if Q > K the reaction will be pushed in the reverse direction, toward the reactants side of the equilibrium, in order to decrease the value of Q. Thus, since the question says the reaction is spontaneous in the reverse direction, Q must be greater than K. This makes choice D the best answer.

Also, note that this question asks about concentration cells, which are not mentioned in the passage, and therefore this problem is essentially a free-standing question.

G-Chem Explicit Questions

Explicit questions require direct retrieval of information from the passage. Sometimes, the answers to Explicit questions are definitions or relationships that are clearly stated in the passage. However, these types of questions may also require some background knowledge or a simple step of logical reasoning. Here is another example from the car battery redox passage shown above:

Of the following, which is the best reducing agent?

A) Li+

B) Li

C) Cl

D) F

To answer this question, you must have fundamental knowledge of redox definitions and relationships, but you also need to retrieve information from the passage. The best reducing agent is the species that has the highest oxidizing potential, and Table 1 gives the reduction potentials for these reagents. However, you also need the knowledge that the oxidation potential is the same as the reduction potential, but with the opposite sign. Since the oxidation of Li has the highest positive potential (3.05 V), Li is the strongest reducing agent.

The best way to approach Explicit questions is to refer to your passage map to find the location of the information you need. Then, go back to the passage and read that section in greater detail. There are two instances when retrieval of information for Explicit questions can be especially tricky. First, in research study passages, be cautious when retrieving information from tables and graphs. Rather than simply pulling data directly from the figures, be sure to read the text just before and after the figures as well, as it may contain important information that changes the way the data should be interpreted. Second, when a passage goes into greater detail about a subject that you already have fundamental knowledge of, avoid the temptation to answer questions directly from memory. Often, these types of passages will provide some obscure detail or anomalous situation that will be tested in the questions, and require you to retrieve information from the passage in order to select the correct answer.

G-Chem Implicit Questions

Implicit questions require you to work through two or more steps of critical reasoning based on your background knowledge and information given in the passage. In other words, the answer is not directly stated in the passage, but is implied by the information provided. The distinction between an Implicit and an Explicit question can be subtle, as both require you to retrieve information from the passage, and Explicit questions may also require you to make a simple critical reasoning decision. The difference is that in Implicit questions, the reasoning step required is not as direct or obvious, and more than one step is usually required. For example:

When a lead storage battery recharges, what happens to the density of the battery fluid?

A) It decreases to 1.0 g/cm3.

B) It increases to 1.0 g/cm3.

C) It decreases to 1.2 g/cm3.

D) It increases to 1.2 g/cm3.

First, information on the density of the battery fluid must be retrieved from the passage. Our passage map tells us that specific information on car batteries can be found in paragraphs two and three. Re-skimming these sections reveals that in the third paragraph of the passage, it states that the density of fluid in a fully charged battery is 1.2 g/cm3. Therefore, as the battery is recharging, its density is approaching this value, eliminating choices A and B.

The difference between choices C and D is whether the density of the solution is increasing or decreasing to 1.2 g/cm3 during recharge. To determine this, we can look for additional information in the passage that may relate to changing density of the battery fluid. The second paragraph of the passage states that as the battery discharges, sulfate ions are consumed and deposited in the form of lead sulfate. The removal of ions from solution implies that the amount of mass in the solution is going down, and therefore its density is also decreasing. Therefore, density is decreasing during discharge, and increasing during recharge. This makes choice D the best answer.

The key step here is focusing on the differences among answer choices. What can be difficult about approaching implicit questions is that it often hard to determine which information is supposed to “imply” something about the answer. Zeroing in on differences among the answer choices can help you determine which information from the passage is most relevant, and may help you rephrase what the question is really asking. Also, note that the first step of our analysis, eliminating the choices with 1.0 g/cm3 density, was basically just answering an explicit question via direct passage retrieval. Many implicit questions begin this way, and it is much easier to eliminate answer choices first based on explicit information than it is to try to make a decision based on implicit information.

2.7 SUMMARY OF THE APPROACH TO GENERAL CHEMISTRY

How to Map the Passage and Use Scratch Paper

1) The passage should not be read like textbook material, with the intent of learning something from every sentence (science majors especially will be tempted to read this way). Skim through the paragraphs to get a feel for the type of questions that will follow, and to get a general idea of the location of information within the passage. For example, you might jot down on your scratch paper that paragraph 1 discusses properties of buffers, and that equation 1 is the Henderson-Hasselbach equation. These observations tip you off that the questions will likely relate to acid/base properties, and that if you need more information about the specific properties of buffers, you can refer to paragraph 1.

2) Highlighting—Use this tool sparingly, or you will end up with a passage that is completely covered in yellow highlighter! Keep in mind that highlighting does not persist as you move from passage to passage within the section. If you want to make more permanent notes, use the scratch paper. Highlighting in a General Chemistry passage should be used to draw attention to a few words that demonstrate one of the following:

• The main theme of a paragraph

• A general chemistry principle that has been taught (i.e., familiar terms)

• An unusual or unfamiliar term that is defined specifically for that passage (e.g., something that is italicized)

3) Pay brief attention to equations, figures, and experiments, noting only what information they deal with. Do not spend a lot of time analyzing at this point, as you can come back and look more closely at this information if a question requires it.

4) Scratch paper is only useful if it is kept organized! Make sure that your notes for each passage are clearly delineated and marked with the passage number on your scratch paper. This will allow you to easily read your notes when you come back to review a marked question. Resist the temptation to write in the first available blank space, as this makes it much more difficult to refer back to your work.

General Chemistry Question Strategies

1) Remember that Process of Elimination is paramount! The strikeout tool allows you to eliminate answer choices; this will improve your chances of guessing the correct answer if you are unable to narrow it down to one choice.

2) Answer the straightforward questions first. Leave questions that require analysis of experiments and graphs for later.

3) Make sure that the answer you choose actually answers the question, and isn’t just a true statement.

4) I-II-III questions: Always work between the I-II-III statements and the answer choices. Unfortunately, it is not possible to strike out the Roman numerals, but this is a great use for scratch paper notes. Once a statement is determined to be true (or false), strike out answer choices that do not contain (or do contain) that statement.

5) LEAST/EXCEPT/NOT questions: Don’t get tricked by these questions that ask you to pick the answer that doesn’t fit (the incorrect or false statement). It’s often good to use your scratch paper and write a T or F next to answer choices A—D. The one that stands out as different is the correct answer!

6) 2 × 2 style questions: These questions require you to know two pieces of information to get the correct answer, and are easily identified by their answer choices, which commonly take the form A because X, B because X, A because Y, B because Y. Tackle one piece of information at a time, which should allow you to quickly eliminate two answer choices.

7) Ranking questions: When asked to rank items, look for an extreme—either the greatest or the smallest item—and eliminate answer choices that do not have that item shown at the correct end of the ranking. This is often enough to eliminate one to three answer choices. Based on the remaining choices, look for the other extreme at the other end of the ranking and use POE again.

8) If you read a question and do not know how to answer it, look to the passage for help. It is likely that the passage contains information pertinent to answering the question, either within the text or in the form of experimental data.

9) If a question requires a lengthy calculation, mark it and return to it later, particularly if you are slow with arithmetic or dimensional analysis.

10) Again, don’t leave any question blank.

A NOTE ABOUT FLASHCARDS

Contrary to popular belief, flashcards are NOT the best way to study for the MCAT. For most of the exams you’ve taken previously, flashcards were probably helpful. This was because those exams mostly required you to regurgitate information, and flashcards are pretty good at helping you memorize facts. Remember, however, that the most challenging aspect of the MCAT is not that it requires you to memorize the fine details of content knowledge, but that it requires you to apply your basic scientific knowledge to unfamiliar situations. Flashcards won’t help you do that.

There is only one situation in which flashcards can be beneficial, and that’s if your basic content knowledge is deficient in some area. For example, if you don’t know the strong acids and bases, flashcards can help you memorize these facts. Or, maybe you are unsure of some of molecular geometries and shapes from the VSEPR theory. You might find that flashcards can help you memorize these. (And remember that part of what makes flashcards useful is that you make them yourself. Not only are they customized for your personal areas of weakness, but the very act of writing down facts on a flashcard helps that information stick in your brain.) But if you are not trying to memorize basic facts in your personal weak areas, you are better off doing and analyzing practice passages than carrying around a stack of flashcards.