molecular mass determination by relative gas density

When the substance is (g), then you have automatically a homogeneous substance; a gas is always homogeneous divided in space.
Here also we have to weigh: a certain volume of gas is weighed at known temperature and pressure.

Then you must weigh a same amount of Hydrogen gas at the same temperature and pressure. You can calculate now how many times heavier the unknown gas is compared with Hydrogen.

att.: the gases X (any unknown gas) and H2 must be weighed at equal temp and pressure.

If you know that the unknown gas is x times heavier that Hydrogen, then we call that number of x: the gas density DH.


DH =    mass of V liter gas (at t and p)
            mass of V liter H2 (at t and p)

Don't forget that for gases at equal t and p always the same volume has the same number of molecules, then you can derive from that definition the following formula:

DH = M(gas)/M(H2)

or rather:

M = 2DH