The vapour density of a gas is the number of times a given volume of the gas is as heavy as the same volume of hydrogen at a particular temperature and pressure.

Since vapour density is a ratio of gas volume, it has no unit. It is related to the relative molecular mass by the formula.

Relative molecular mass = 2 × vapour density mass

Or

Vapour density = \( \frac{1}{2} \scriptsize \: \times \: Relative \: molecular \: mass \)

### Example 6.6.1:

Calculate the vapour density of sulphur (IV) oxide (SO_{2}) (S = 32, O = 16)

**Solution:**

Molar mass of SO_{2} = 32 + 16 × 2 = 64

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