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 H_{2} 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 D_{H}.
Definition:
D_{H} = mass of V liter gas (at t and p)
mass of V liter H_{2} (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: