Tim,

You need to know the density of the gas. If you mean "gasoline" it is about 750g/l ; if you mean "a gas" in the sense of vapor, one "mole" of a pure (one type of molecule) gas takes up 22.4 liters at standard temperature (0 C) and pressure (sea level). A cubic foot is about 28.3 liters, or about 1.26 moles (one and a quarter will do for most purposes.)

A mole is a quantity corresponding to 6.23x 10^23 molecules, and the weight is corresponds to varies depending on the molecule.

To find this weight you need to know the molecular formula of the gas,and the atomic weight of every type of atom in it. This is given in most periodic tables, and is approximately the isotope number. Add these, multiplying each by the number of occurrences in the molecule. Thus:

Nitrogen, N2: w(N) = 14, M = 2x14 = 28 grams per mole. So a cubic foot of it weighs 1.26 * 28 grams or about 35 grams. This is also a good approximation for air, which is mostly nitrogen.

Carbon dioxide, CO2: w(C) = 12, w(O) = 16, M= 12+2*16 = 44.
So a cubic foot weighs 55 grams.

If your pressure or temperature are nonstandard, you need to correct. At P atmospheres pressure, multiply by P; at a temperature T (in Kelvin: this is Celsius + 273) multiply by 273 and divide by T.

Good hunting!

-RD