SAT Subject Test Chemistry

PART 2

REVIEW OF MAJOR TOPICS

CHAPTER 4

Chemical Formulas

These skills are usually tested on the SAT Subject Test in Chemistry. You should be able to . . .

• Recall and use the basic rules about oxidation states to write correct formulas. This includes writing formulas with polyatomic ions.

• Name compounds (acids, bases, and salts) using the Stock system and the prefix system, and write their formulas.

• Calculate the oxidation states of an element in any formula.

• Calculate the formula mass of a compound and the percent composition of each element.

• Calculate the empirical formula when given the percent composition of each element. When given the formula mass, you should be able to find the true formula.

• Write a simple balanced equation, indicating the phase (or state) of the reactants and products.

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

NAMING AND WRITING CHEMICAL FORMULAS

With the knowledge you have about atomic structure, the significance of each element’s placement in the periodic table, and the bonding of atoms in ionic and covalent arrangements, you can now use this information to write appropriate formulas and name the resulting products. Obviously, many compounds can result. Some system of writing the names and formulas of these many combinations was needed. The system explained in this text is an organized way of accomplishing this. It uses three categories:

CATEGORY I—Binary ionic compounds where the metal present forms only a single type of positively charged ion (cation)

CATEGORY II—Binary ionic compounds where the metal forms more than one type of ionic compound with a given negatively charged ion (anion)

CATEGORY III—Binary covalent compounds formed between two nonmetals 

Table 6 is a list of ions that are often encountered in a first-year chemistry course. You should know them. Although using the period table can help you write the symbol and apparent charge of cations and anions, knowing these common ions can help you write formulas and equations.

Category I—Binary Ionic Compounds

Category I binary ionic compounds are metallic ions from groups 1 and 2 of the Periodic Table. These metallic ions have only one type of charge. The binary ionic compounds formed are composed of a positive ion (cation) that is written first and a negative ion (anion). The following rules show how to name and write the formulas for binary ionic compounds. CaClis used as an example.

  1. Name the cation first and then the anion.

  2. The monoatomic (one-atom) cation takes its name from the name of the element. Therefore the calcium ion, Ca2+, is called calcium and its chemical symbol appears first.

  3. The monoatomic anion with which the cation combines is named by taking the root of the element’s name and adding –ide. You must know this rule. The anion’s name comes second. Therefore, the chlorine ion, Cl, is called chloride.

  4. The name of this compound is calcium chloride.

A quick way to determine the formula of a binary ionic compound is to use the crisscross rule.

EXAMPLE 1: To determine the formula for calcium chloride, first write the ionic forms with their associated charges.

Next move the numerical value of the metal ion’s superscript (without the charge) to the subscript of the nonmetal’s symbol. Then take the numerical value of the nonmetal’s superscript and make it the subscript of the metal as shown above.

Note that the numerical value 1 is not shown in the final formula.

You now have the chlorine’s 1 as the subscript of the calcium and the calcium’s 2 as the subscript of the chloride. As a result, you have CaClas the final formula for calcium chloride.

EXAMPLE 2:  Write the name and formula for the product formed when aluminum reacts with oxygen. First write the name.

1. Name the cation first and then the anion.

2. The monoatomic (one-atom) cation takes its name from the name of the element. Therefore the aluminum ion, Al3+, is called aluminum and its chemical symbol appears first.

3. The monoatomic anion with which the cation combines is named by taking the root of the element’s name and adding –ideYou must know this rule. The anion’s name comes second. Therefore the oxygen ion, O2−, is called oxide.

4. The name of this compound is aluminum oxide.

To determine the formula for aluminum oxide, first write the ionic forms with their associated charges.

Next move the numerical value of the Al’s superscript (without the charge) to the subscript of the O symbol. Do the same with the 2 of the O. In other words, crisscross the values. You now have the 2 as the subscript of the aluminum and the 3 as a subscript of the oxygen. You now have Al2Oas the final formula for aluminum oxide.

This crisscross rule generally works very well. In one situation, though, you have to be careful. Suppose you want to write the compound formed when magnesium reacts with oxygen. Magnesium, an alkaline earth metal in group 2 forms a 2+ cation, and oxygen forms a 2 anion. You would predict its formula be Mg2O2, but this is incorrect. After you do the crisscrossing (unless you know that the compound actually exists, like H2O2), you need to reduce all the subscripts by a common factor. In this example, you can divide all the subscripts by 2 to get the correct formula for magnesium oxide, MgO.

When you attempt to write a formula, you should know whether the substance actually exists. For example, you could easily write the formula for carbon nitrate, but no chemist has ever prepared this compound.

TIP 

REMEMBER Reduce all subscripts by a common factor unless you are sure the compound exists, like H2O2.

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* Ionic charges are shown as numerical exponents followed by the charge.

Category II—Binary Ionic Compounds

In category II binary ionic compounds, the metals form more than one ion, each with a different charge. The metallic ions (cation) ionically bind with a negatively charged ion (anion).

The following chart lists most of the metals that form more than one type of ionic cation and therefore more than one binary ionic compound with a given anion.

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* This form of mercury(I) ions always occurs bonded together as a Hg22+ ion.

Although the following metals are “transition” metals, they form only one type of cation. So a Roman numeral is not used when naming their compounds.

Ag1+     Silver

Cd2+     Cadmium

Zn2+      Zinc

EXAMPLE:   The compound containing the Fe2+ ion and the compound containing the Fe3+ ion both combine with the chloride ion to form two different compounds. Using the crisscross system, you get the formula FeClfor iron(II) chloride.

The compound formed using the Fe3+ ion and the chloride ion is FeCl3, which is iron(III) chloride.

The names iron(II) chloride and iron(III) chloride are arrived at by using the Roman numerals in parentheses to indicate the charge of the metallic ion used as the cation.

Another, older system of naming category II binary ionic compounds is still seen in some books. Simply stated, for metals that form only two ions, the ion with the higher charge has a name ending in –ic and the ion with the lower charge has a name ending in –ous. In this system, Fe3+ is called the ferric ion and Fe2+ is called the ferrous ion. The names for FeCland FeClare then ferric chloride and ferrous chloride, respectively.

Examples of Category II Binary Ionic Compounds

Formula

        Name

CuCl

Copper(I) chloride

HgO*

Mercury(II) oxide

FeO*

Ferrous(II) oxide

MnO2

Manganese(IV) oxide

PbCl2

Lead(II) chloride

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* The subscripts are reduced and are not written because subscripts of 1 are understood.
 The subscripts are reduced.

The modified periodic chart that follows shows the location of the common category I and category II ions. Also shown in this chart are the common nonmetallic monoatomic ions as anions.

Category I and II Ionic Compounds Formed with Polyatomic Ions

Another group of ionic compounds contains polyatomic ions. A polyatomic ion is a group of elements that act like a single ion when forming a compound. The bonds within these polyatomic ions are predominately covalent. However, the group as a whole has an excess charge, which is usually negative, because of an excess of electrons. If the compounds formed with the polyatomic ions consist of three elements, they are called ternary compounds.

TIP 

You should memorize all the polyatomic ions in Table 6 to help you use them in formulas and equations.

Polyatomic ions have special names and formulas that you must memorize. Table 6 contains the names and ionic charges of the common polyatomic ions encountered in a first-year chemistry course. Note that only one commonly used positively charged polyatomic ion is in Table 6, the ammonium ion, NH4+.

Also notice in Table 6 that several of the polyatomic anions contain an atom of a given element and a different number of oxygen atoms, such as NOand NO3. When there are two members of such a series, the name of the one with fewer oxygen atoms ends in -ite and the name of the one with more oxygen atoms ends in –ate. The following table shows examples of polyatomic ions of sulfur.

Sometimes an element combines with oxygen to form more than just two polyatomic ions, such as ClO, ClO2, ClO3, and ClO4. When this occurs, the prefix hypo- is used to name the polyatomic ion with the fewest oxygen ions and the prefix per- to name the polyatomic ion with the most oxygen ions.

Writing Formulas with Polyatomic Ions

When writing formulas using polyatomic anions, the rules do not change. Simply treat the polyatomic ion as if it were a single anion. If the cation is from category I, follow the rules for category I. If the cation is from category II, follow the rules for category II. The crisscross method does not change, either.

EXAMPLE 1: Use the crisscross method to write the formula for calcium sulfate, a category I cation and a polyatomic ion.

Calcium sulfate

The final formula is CaSO4. Notice that the subscripts “2” are omitted.

EXAMPLE 2: Use the crisscross method to write the formula for iron(III) sulfate, a category II cation and a polyatomic ion.

Iron(III) sulfate

The final formula is Fe2(SO4)3.

Category III—Binary Covalent Compounds

Binary covalent compounds are formed between two nonmetals. Although these compounds do not contain ions, they are named very similarly to binary ionic compounds. To name binary covalent compounds, use these steps.

  1. The first element in the formula is named first, using the full elemental name.

  2. The second element is named as if it were an anion and uses its elemental name.

  3. Prefixes are used to denote the number of the second element present. These prefixes are shown in the table below.

  4. The prefix mono- is never used for naming the first element. For example, CO is called carbon monoxide, not monocarbon monoxide.

The following are examples of covalent compounds formed from the nonmetals nitrogen and oxygen, using the rules above.

Compound

Systematic Name

Common Name

N2O

Dinitrogen monoxide*

Nitrous oxide

NO

Nitrogen monoxide*

Nitric oxide

NO2

Nitrogen dioxide

 

N2O3

Dinitrogen trioxide

 

N2O4

Dinitrogen tetroxide*

 

N2O5

Dinitrogen pentoxide*

 

* Notice that for ease of pronunciation, the final “a” or “o” of the prefix is dropped if the element begins with a vowel.

To write the formula for binary covalent compounds, use the same steps as when writing the formula of ionic compounds.

  1. The symbol of the first element in the formula is written first, followed by the second element.

  2. Use the prefix(es) denoted in the name for the number of each element present in the formula.

The following show some examples of binary covalent compounds.

Name

Formula

Sulfur hexafluoride

SF6

Phosphorus trichloride

PCl3