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. The standard conditions of temperature and pressure (abbreviated STP) are 0 degrees Celsius 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 ration to each other as small whole numbers. This law may be illustrated by the following cases:

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

2 vols. hydrogen + 1 vol. oxygen = 2 vols. steam

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 liter, 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

3H2 (3 moles H2) = 6g = 67.2 liters