A gas occupies 3 litres at 0°C. What volume will it occupy at -20°C pressure remaining constant?

V₁ = Initial volume of the gas = 3 litres

T₁ = Initial temperature of the gas = 0°C = 273 K

T₂ = Final temperature of the gas = -20°C = -20 + 273 = 253 K

V₂ = Final volume of the gas = ?

By Charles' Law:

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

Substituting the values :

$\dfrac{3}{273} = \dfrac{\text{V}$2}{253} \\[1em] \text{V}2 = \dfrac{3 \times 253}{273} \\[1em] \text{V}_2 = 2.78 \text{ litres}

∴ Final volume = 2.78 litres.