At constant temperature 200 cm³ of N₂ at 720 mm and 400 cm³ of O₂ at 750 mm pressure are put together in a one litre flask. The final pressure of mixture is

The number of moles of nitrogen,$\mathrm{n}= \frac{\mathrm{PV}}{\mathrm{RT}} = \frac{720\times 200}{\mathrm{RT}}$

The number of moles of oxygen,$\mathrm{n}=\frac{\mathrm{PV}}{\mathrm{RT}}=\frac{750\times 400}{\mathrm{RT}}$

The final pressure, $$\begin{gathered} \mathrm{P}= \frac{\mathrm{nRT}}{\mathrm{V}}= \frac{ {\displaystyle \frac{720\times 200}{\mathrm{RT}}} +{\displaystyle \frac{750\times 400}{\mathrm{RT}}}}{1000} \mathrm{RT} \\ =\frac{144000+300000}{1000} =444\mathrm{mm} \end{gathered}$$