SAT Subject Test Chemistry

PART 2

REVIEW OF MAJOR TOPICS

CHAPTER 6

Stoichiometry (Chemical Calculations) and the Mole Concept

GAS VOLUMES AND MOLAR MASS

Because the volume of a gas may vary depending on the conditions of temperature and pressure, a standard is set for comparing gases. As stated in Chapter 5, the standard conditions of temperature and pressure (abbreviated STP) are 273 K and 760 mm of mercury pressure.

The molecular mass of a gas expressed in grams and under standard conditions occupies 22.4 liters. This is an important relationship to remember! The 22.4 liters is referred to as the gram-molecular volume (gmv) or molar volume. Two scientists are associated with this relationship.

Gay-Lussac’s Law states that, when only gases are involved in a reaction, the volumes of the reacting gases and the volumes of the gaseous products are in a small-whole-number ratio to each other. This law may be illustrated by the following cases:

Example 1: H2(g) + Cl2(g) → 2HCl(g)

This balanced equation shows that

1 vol. hydrogen + 1 vol. chlorine = 2 vols. hydrogen chloride 

Example 2: 2H2(g) + O2(g) → 2H2O(g)

This balanced equation shows that 2 vols. hydrogen + 1 vol. oxygen = 2 vols. steam

TIP 

Know Gay-Lussac’s Law of combining gases.

Avogadro’s Law, which explains Gay-Lussac’s, states that equal volumes of gases under the same conditions of temperature and pressure contain equal numbers of molecules. This means that 1 mole of any gas at STP occupies 22.4 liters, so:

32 g Oat STP occupies 22.4 liters

  2 g Hat STP occupies 22.4 liters

44 g COat STP occupies 22.4 liters

  2O(2 moles O2) = 64 g = 44.8 liters at STP

  3H(3 moles H2) = 6 g = 67.2 liters at STP