50 cm³ of hydrogen is collected over water at 17°C and 750 mm Hg pressure. Calculate the volume of dry gas at S.T.P. The water vapour pressure at 17°C is 14 mm Hg.

Initial conditions :

P₁ = 750 - water vapour pressure = 750 - 14 = 736 mm Hg

V₁ = Initial volume of the gas = 50 cm³

T₁ = Initial temperature of the gas = 17°C = 17 + 273 = 290 K

Final conditions :

P₂ (Final pressure) = 760 mm of Hg

T₂ (Final temperature) = 273 K

V₂ (Final volume) = ?

By Gas Law:

$\dfrac{\text{P}$1\times\text{V}1}{\text{T}1} = \dfrac{\text{P}2\times\text{V}2}{\text{T}2}

Substituting the values :

$\dfrac{736 \times 50}{290} = \dfrac{760 \times \text{V}$2}{273} \\[1em] \text{V}2 = \dfrac{736 \times 50 \times 273}{290 \times 760} \\[1em] \text{V}_2 = 45.58 \text{ cm}^3 \\[1em]

∴ Volume of dry gas = 45.6 cm³