A vessel of 120mL capacity contain a certain mass of gas at  ${20}^{0}C$ and 750 mm pressure. The gas was transferred to a vessel whose volume is 180mL. Calculate the pressure of the gas at ${20}^{0}C.$   

Since a gas completely fills the vessel in which it is kept, we have
$\begin{array}{l}{V}_1=120mL,V_2=180mL\\ P_1=750mm,P_2=?mm\end{array}$
Since temperature remains constant, by applying Bolye’s law, we get
$P_1{V}_1=P_2{V}_2$
Substituting the corresponding values, we have
$\begin{array}{l}750\times 120=P_2\times 180\\ P_2=\frac{750\times 120}{180}=500mm\\ \end{array}$
Therefore, the pressure of the gas is 500 mm.