At constant temperature, the pressure of a gas is 940 mm of Hg. Calculate the pressure if the volume of the gas in decreased by 20%.

V₁ = Initial volume of the gas = V

P₁ = Initial pressure of the gas = 940 mm of Hg

V₂ = Final volume of the gas = decreased by 20%
= 80% of V₁ = $\dfrac{80}{100}$V = $\dfrac{4}{5}$V

P₂ = Final pressure of the gas = ?

By Boyle's Law:

$\text{P}$1 \text{V}1 = \text{P}2 \text{V}2

Substituting the values :

$\text{V} \times 940 = \text{P}$2 \times \dfrac{4}{5}\text{V} \\[0.5em] \text{P}2 = \dfrac{5\times 940}{4} \\[0.5em] \text{P}_2 = 1175 \text{ mm Hg}

∴ Final pressure = 1175 mm Hg