How do you calculate molecular weight of a gas?

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Answer 1 · 2016-06-03T17:09:58

$M M = \frac{m R T}{P V}$ $MM=(mRT)/(PV)$

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

$P V = n R T$ $PV=nRT$

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

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

$n = \frac{m}{M M}$ $n=m/(MM)$

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

$P V = \frac{m}{M M} R T$ $PV=m/(MM)RT$

therefore, the molecular mass expression can be written as:

$M M = \frac{m R T}{P V}$ $MM=(mRT)/(PV)$

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