Two vessels containing gases A and B are interconected as  shown in the figure. The stopper is opened, the gases are allowed to mix homogeneously. The partial pressures of A and B in the mixture will be, respectively 

 

$$\begin{gathered} \text{ Moles of }\mathrm{A},\left({\mathrm{n}}_{\mathrm{A}}\right)=\frac{{\mathrm{p}}_{\mathrm{A}}{\mathrm{v}}_{\mathrm{A}}}{\mathrm{RT}}=\frac{8\times 12}{\mathrm{RT}}=\frac{96}{\mathrm{RT}} \\ \text{ Moles of }\mathrm{B},\left({\mathrm{n}}_{\mathrm{B}}\right)=\frac{{{\mathrm{p}}_{\mathrm{B}}}^{\mathrm{V}}\mathrm{B}}{\mathrm{RT}}=\frac{8\times 5}{\mathrm{RT}}=\frac{40}{\mathrm{RT}} \\ \text{ Total pressure }\times \text{ total volume }=\left({\mathrm{n}}_{\mathrm{A}}+{\mathrm{n}}_{\mathrm{B}}\right)\times \mathrm{RT} \\ \mathrm{p}\times (12+8)=\frac{1}{\mathrm{RT}}(96+40)\mathrm{RT} \\ \mathrm{p}=6.8 \\ \text{ Partial pressure of }\mathrm{A}=\mathrm{p}\times \text{ mole fraction of }\mathrm{A} \\ =6.8\left(\frac{96}{\mathrm{RT}}/\frac{96+40}{\mathrm{RT}}\right) \\ =4.8\mathrm{atm} \\ \mathrm{Partial} \mathrm{pressure} \mathrm{of} \mathrm{B} = 6.8 — 4.8 = 2 \mathrm{atm}. \end{gathered}$$