What will be the volume of a gas when 3 litres of it is cooled down from 27°C to -73°C at constant pressure?

V₁ = Initial volume of the gas = 3 litres
T₁ = Initial temperature of the gas = 27°C = 27 + 273 = 300 K
T₂ = Final temperature of the gas = -73°C = -73 + 273 = 200 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}{300} = \dfrac{\text{V}$2}{200} \\[1em] \text{V}2 = \dfrac{3 \times 2}{3} \\[1em] \text{V}_2 = 2 \text{ lit}

∴ Final volume = 2 litres.