SAT Subject Test Chemistry

PART 2

REVIEW OF MAJOR TOPICS

CHAPTER 6

Stoichiometry (Chemical Calculations) and the Mole Concept

MOLAR MASS AND MOLES

When the formula mass of an ionic compound is determined by the addition of its component relative atomic masses and expressed in grams, it is called the molar mass.

EXAMPLE: Find the molar mass of CaCO3.

The molar mass is equivalent to the mass of 1 mole of that compound expressed in grams.

ANOTHER EXAMPLE: Find the mass of 1 mole of KAl(SO4)2 · 12H2O.

                    

TRY THESE PROBLEMS: Find the molecular mass or formula mass of each of the following:

1. HNO           Ans. 63 g/mol

2. C6H10O       Ans. 162 g/mol

3. H2SO          Ans. 98 g/mol

4. KCl               Ans. 74.5 g/mol

5. C12H22O11    Ans. 342 g/mol

In some cases, there may be a question of the mass of 1 mole of an element if you are not told specifically what is referred to—the single atoms or a molecular state. An example of this situation may arise with hydrogen and other elements that form diatomic molecules. If you are asked the mass of 1 mole of hydrogen, you could say it is 1 gram if you are dealing with single atoms of hydrogen. This 1 mole of hydrogen atoms would contain 6.02 × 1023 atoms. In a similar fashion, 1 mole of hydrogen molecules (H2) would have a molar mass of 2 grams. This 1 mole of hydrogen molecules would contain 6.02 × 1023 molecules and, since each molecule is composed of two atoms, 12.04 × 1023 atoms.

The other elements that exist as diatomic molecules are oxygen, nitrogen, fluorine, chlorine, bromine, iodine, and astatine.

Examples:

1. What is the mass of 1 mole of oxygen?

Answer: Since the atomic mass of oxygen is 16 grams and the molecule is

diatomic, 2 × 16.0 grams = 32.0 grams.

2. What is the mass of one molecule of oxygen?

Answer: There are 6.02 × 1023 molecules in 1 mole of this gas. 32.0 grams ÷ 6.02×1023 molecules = 5.32 × 10−23gram.

3. What is the mass of 1 mole of oxygen atoms?

Answer: Without the diatomic molecule structure, the mass of one mole of oxygen

atoms expressed in grams is the answer. That is 16.0 g/mol.

4. What is the mass of one atom of oxygen?

Answer: The atomic molar mass, 16.0 grams, is divided by 6.02 × 1023. This

gives 2.66 × 10−23 gram.

TIP 

Know how to use these relationships!

If you gave the same answer to the first two questions, you are confused about the mole concept. To say that a molecule of oxygen gas has the same mass as a mole of oxygen gas is the same as saying that an apple has the same mass as a dozen apples.