The Foundations of Chemistry

The volume occupied by a mole of gas at standard temperature and pressure,STP, is
referred to as the standard molar volume. It is nearly constant for all gases (Table 12-3).

The standard molar volumeof an ideal gas is taken to be 22.414 liters per mole

at STP.

Gas densities depend on pressure and temperature; however, the number of moles of
gas in a given sample does not change with temperature or pressure. Pressure changes
affect volumes of gases according to Boyle’s Law, and temperature changes affect volumes
of gases according to Charles’s Law. We can use these laws to convert gas densities at
various temperatures and pressures to standard temperature and pressure.Table 12-3 gives
the experimentally determined densities of several gases at standard temperature and
pressure.

EXAMPLE 12-6 Molecular Weight, Density

One (1.00) mole of a gas occupies 27.0 liters, and its density is 1.41 g/L at a particular temper-
ature and pressure. What is its molecular weight? What is the density of the gas at STP?

Plan

We can use dimensional analysis to convert the density, 1.41 g/L, to molecular weight, g/mol.
To calculate the density at STP, we recall that the volume occupied by one mole would be
22.4 L.

Solution

We multiply the density under the original conditions by the unit factor 27.0 L/1.00 mol to
generate the appropriate units, g/mol.

38.1 g/mol

At STP, 1.00 mol of the gas, 38.1 g, would occupy 22.4 L, and its density would be

Density1.70 g/L at STP

You should now work Exercises 40 and 42.

1 mol



22.4 L

38.1 g



1 mol

27.0 L



mol

1.41 g



L

_?_g



mol

The volume percentages given in

Table 12-2 are also equal to mole

percentages.

Density is defined as mass per unit

volume.

12-8 Avogadro’s Law and the Standard Molar Volume 449

TABLE 12-3 Standard Molar Volumes and Densities of Some Gases

Standard Molar Density at

Gas Formula (g/mol) Volume (L/mol) STP (g/L)

hydrogen H 2 2.02 22.428 0.090
helium He 4.003 22.426 0.178
neon Ne 20.18 22.425 0.900
nitrogen N 2 28.01 22.404 1.250
oxygen O 2 32.00 22.394 1.429
argon Ar 39.95 22.393 1.784
carbon dioxide CO 2 44.01 22.256 1.977
ammonia NH 3 17.03 22.094 0.771
chlorine Cl 2 70.91 22.063 3.214

Deviations in standard molar volume

indicate that gases do not behave

ideally.