SAT Subject Test Chemistry
REVIEW OF MAJOR TOPICS
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
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 O2 at STP occupies 22.4 liters
2 g H2 at STP occupies 22.4 liters
44 g CO2 at STP occupies 22.4 liters
2O2 (2 moles O2) = 64 g = 44.8 liters at STP
3H2 (3 moles H2) = 6 g = 67.2 liters at STP