Let us consider a gas X having molar mass
Now if the actual mass of
Then actual masss of a molecule of X gas will be
So one gm mole or
Hence from this discussion we can say 1 gm mole of any element or compound (here the gas considered) will always contain
Now by Avogadro's hypothesis we know, under the same condition of temperature and prssure same volume of all gases contain same number of molecules
In reverse statement of this law we can say that same number of gaseous molecules will always occupy same volume under same condition of temperature and pressure (say STP).
So one gram mole of any gas which contains
So volume of one mole of gas is fixed at STP.This volume is known as molar volume of gas at STP.