SAT Subject Test Chemistry

PART 2

REVIEW OF MAJOR TOPICS

CHAPTER 6

Stoichiometry (Chemical Calculations) and the Mole Concept

These skills are usually tested on the SAT Subject Test in Chemistry. You

should be able to . . .

• Use the mole concept to find the molar mass of formulas and of monoatomic and diatomic molecules, and how gas volumes are related to molar mass.

• Solve stoichiometric problems involving Gay-Lussac’s Law, density, mass–volume relation, mass–mass problems, volume–volume problems, problems with a limiting reactant (excess of one reactant), and finding the percent yield.

This chapter will review and strengthen these skills. Be sure to do the Practice Exercises at the end of the chapter.

This chapter deals with the solving of a variety of chemistry problems, which is often referred to as stoichiometry. Solving problems should be done in an organized manner, and it would be to your advantage to go back to the Introduction to this book and review the section called, “How Can You Improve Your Problem Solving Skills?” It describes a well-planned method for attacking the process of solving problems that you will find helpful in this chapter.

Two or three methods of solving the problems in this chapter are usually given. It would be wise on your part to learn the mole method, the proportion method, and the factor-label method (also referred to as dimensional analysis), so that you have more tools available for solving problems. Remember that no matter which method you use, you should still use the estimation process to verify the plausibility of the answer.

THE MOLE CONCEPT

Providing a name for a quantity of things taken as a whole is common in everyday life. Some examples are a dozen, a gross, and a ream. Each of these represents a specific number of items and is not dependent on the commodity. A dozen eggs, oranges, or bananas will always represent 12 items.

TIP 

Know Avogadro’s number and its use.

In chemistry we have a unit that decribes a quantity of particles. It is called the mole (sometimes abbreviated as mol ). A mole is 6.02 × 1023 particles. The particles can be atoms, molecules, ions, electrons, and so forth. Because particles are so small in chemistry, the mole is a very convenient unit. The number 6.02 × 1023 is often referred to as Avogadro’s number in honor of the Italian scientist whose hypothesis led to its determination.

In 1811, Amedeo Avogadro made a far-reaching scientific assumption (hypothesis) that also bears his name. He stated that equal volumes of different gases contain equal numbers of molecules at the same pressure and temperature. The statement is called Avogadro’s Law. It means that, under the same conditions, the number of molecules of hydrogen in a 1-liter container is exactly the same as the number of molecules of carbon dioxide or any other gas in a 1-liter container even though the individual molecules of the different gases have different masses and sizes. Because of the substantiation of this hypothesis by many data since its inception, it is often referred to as Avogadro’s Law.