## Definition of molar volume

Molar volume (denoted by Vm) is the volume of one mole of a gas at a given temperature and pressure.

However, volume of a gas depends upon the conditions of pressure and temperature. Therefore, if the conditions of temperature and pressure are changed the molar volume will also change. Thus it is customary to use the molar volume at standard temperature and pressure (STP).

### Molar volume at STP

It has been found experimentally that one mole of any gas at STP occupies a volume of 22.4 dm3. This is called molar volume of gases at STP.

### Uses of molar volume

The information that molar volume at STP of any gas is 22.4 dm3 is particularly important for knowing the number of moles, number of particles (i-e, atoms or molecules) and mass of the gas in a certain volume.

Number of moles can be calculated by dividing the given volume on the molar volume. Mathematically, if there is one mole in 22.4 dm3, there would be 2 moles in 44.8 dm3 at STP. Similarly, 0.5 moles in 11.2 dm3 at STP. This means, at STP,

Number of moles = (given volume)/22.4

Moreover, knowing the number of moles, number of particles can be calculated by multiplying the number of moles by 6.023 × 1023.

Knowing number of moles also makes possible to find the mass of the gas contained the given volume.

(b) Numerical Part

(i) To find volume of 2.5 moles of H2 at STP.

We know 1 mol occupies 22.4 dm3 of space (volume) at STP, the 2.5 moles will occupy 2.5 × 22.4 = 56 dm3 volume.

(ii) We should know the gram formula mass of NH3. It is 14 + 3 = 17 g.

Now 17 g of NH3 = 1 mol —–→ 1 g of NH3 = 1/17 moles —–→ 60 g = 1/17 × 60 = 3.53 moles

Now apply the molar volume at STP concept,

1 mole occupies volume at STP = 22.4 dm3

3.53 moles of NH3 occupies volume at STP = 22.4 × 3.53 dm3 = 79.05 dm3