# How do you calculate molecular weight of a gas?

Jun 3, 2016

$M M = \frac{m R T}{P V}$

#### Explanation:

To find the molecular mass of a gas, we can assume it behaves ideally and use the ideal gas law:

$P V = n R T$

We can modify this law in terms of the molar mass ($M M$).

We know that the number of mole is equal to the mass ($m$) divided by th molecular mass ($M M$) of the gas:

$n = \frac{m}{M M}$

If we plug the expression of number of mole in the expression of the ideal gas law we get:

$P V = \frac{m}{M M} R T$

therefore, the molecular mass expression can be written as:

$M M = \frac{m R T}{P V}$

Thus, to find the molecular mass of a gas, we can measure its pressure, volume, mass and temperature.