# Question 207bb

Aug 25, 2016

Let us consider a gas X having molar mass $= M \frac{g}{\text{mol}}$

Now if the actual mass of $\frac{1}{12}$th part of a ""_6^12C# atom be $\text{ " A g " }$which is a constant.

Then actual masss of a molecule of X gas will be $= M A g$

So one gm mole or $M g \text{ }$ gas will contatin $\frac{M}{M A} = \frac{1}{A}$molecules of the gas.

Hence from this discussion we can say 1 gm mole of any element or compound (here the gas considered) will always contain $\frac{1}{A}$,a definite number of molecules.

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 $\frac{1}{A}$ (a definite) molecules should also occupy a definite volume at STP and that volume is shown to be $22.414 {\mathrm{dm}}^{3}$ by calculation.

So volume of one mole of gas is fixed at STP.This volume is known as molar volume of gas at STP.