If we keep the temperature and pressure of a gas constant the volume is proportional to the number of moles or its mass, g, divided by the molecular weight.

V = a*g/MW

Let's do an experiment in which we also keep the volume constant and compare the mass and molecular weight of different gases.

If V doesn't change we have a new constant a/V which we can call c.

If T, P, and V are constant:

MW =c*g

This says tha the molecular weight of a gas is directly proportional to its mass if we keep T, V, and P constant.

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