**Question 5:** Define molar volume of a gas. What will be the volume of 2.5 moles of H_{2} gas and 60 g NH_{3} at STP?

**Answer**

## Definition of molar volume

Molar volume (denoted by V_{m}) 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 dm^{3}. 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 dm^{3} 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 dm^{3}, there would be 2 moles in 44.8 dm^{3} at STP. Similarly, 0.5 moles in 11.2 dm^{3} 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 × 10^{23}.

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 H _{2 }at STP.**

We know 1 mol occupies 22.4 dm^{3} of space (volume) at STP, the 2.5 moles will occupy 2.5 × 22.4 = **56 dm ^{3}** volume.

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

Now 17 g of NH_{3} = 1 mol —–→ 1 g of NH_{3} = 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 dm^{3}

3.53 moles of NH_{3} occupies volume at STP = 22.4 × 3.53 dm^{3} = **79.05 dm ^{3}**