Molar Mass of Gases and Gas Mixtures
Molar mass (M) is equal to the mass of one mole of a particular element or compound; as such, molar masses are expressed in units of grams per mole (g mol^{–1}) and are often referred to as molecular weights. The molar mass of a particular gas is therefore equal to the mass of a single particle of that gas multiplied by Avogadro's number (6.02 x 10^{23} ). For a mixture of gases, the molar mass depends on the molar masses of each of its components and on the fractional abundance of each kind of gas in the mixture. In this case, the term "average molecular weight" is often used to describe the average molar mass of the gas mixture.
The average molar mass of a mixture of gases is equal to the sum of the mole fractions of each gas, multiplied by their respective molar masses:
The molar volumes of all gases are the same when measured at the same temperature and pressure (22.4 L at STP), but the molar masses of different gases will almost always vary.
Calculating Molar Mass using the Ideal Gas Equation
The molar mass of an ideal gas can be determined using yet another derivation of the Ideal Gas Law:
We can write n, number of moles, as follows:
where m is the mass of the gas, and M is the molar mass. We can plug this into the Ideal Gas Equation:
Rearranging, we get:
Finally, putting the equation in terms of molar mass, we have:
This derivation of the Ideal Gas Equation is useful in determining the molar mass of an unknown gas.
Example
 An unknown gas with a mass of 205 g occupies a volume of 20.0 L at standard temperature and pressure. What is the molar mass of the gas?

$M=\frac{mRT}{PV}$