1000 ml of a gas A at 600 torr and 500 ml of gas B at 800 torr are placed in a 2 L flask. The final pressure will be 

From Dalton's partial pressure law

$$\begin{gathered} {\mathrm{P}}_1=\frac{1000}{1500}\times 600+\frac{500}{1500}\times 800 \\ =400 + \frac{800}{3}=\frac{2000}{3}\mathrm{torr}. \\ {\mathrm{V}}_1=1500 \mathrm{mL} \\ {\mathrm{P}}_2 = ? {\mathrm{V}}_2 = 2\mathrm{L} = 2000 \mathrm{mL} \\ \mathrm{From} \mathrm{P} \propto \frac{1}{\mathrm{V}} \\ \frac{{\mathrm{P}}_1}{{\mathrm{P}}_2}=\frac{{\mathrm{V}}_2}{{\mathrm{V}}_1}\Rightarrow \frac{2000/3}{{\mathrm{P}}_2}=\frac{2000}{1500} \\ \Rightarrow {\mathrm{P}}_2=\frac{1500}{3}=500 \mathrm{torr} \end{gathered}$$