A vessel of volume $33.2\times {10}^{-3}{m}^3$ contains an ideal gas at 300K and 200 kPa. The gas is  allowed to leak until the pressure falls to 125 kPa. Calculate the amount of the gas  (in moles) leaked assuming that the temperature remains constant.

As he gas leaks out, the volume and the temperature of the remaining gas do not  change. The number of moles of the gas in the vessel is given by  $n=\frac{PV}{RT}$. 
The number of moles in the vessel before the leakage is $n_1=\frac{P_{1}V}{RT}$.
The number of moles in the vessel after the leakage is $n_2=\frac{P_{2}V}{RT}$.
Thus, the number of moles leaked is  $n_1-n_2=\frac{\left(P_1-P_2\right)V}{RT}$
$=\frac{\left(200-125\right)\times {10}^3\times \left(33.2\times {10}^{-3}\right)}{8.3\left(300\right)}=1mole$.