A gaseous mixture containing 0.35g of ${\mathrm{N}}_2$ and 5600 ml of ${\mathrm{O}}_2$ at STP is kept in a 5 litres flask at 300K. The total pressure of the gaseous mixture is

$\text{ Partial pressure of }{\mathrm{N}}_2=\frac{\mathrm{nRT}}{\mathrm{V}}=\frac{\left(\frac{0.35}{28}\right)\times 0.0821\times 300}{5}=0.061\mathrm{atm}$${\mathrm{O}}_2=5600\mathrm{ml}$ Molecular weight of  ${\mathrm{O}}_2=32\mathrm{gr}32\mathrm{gr}⟶22.4\mathrm{L}=22,400\mathrm{ml}$

$\begin{array}{rr} & \mathrm{ }?⟵5,600\mathrm{ml}=\frac{32\times 5600}{22,400}=8\mathrm{gr}\text{ Partial pressure of }{\mathrm{O}}_2=\frac{\left(\frac{8}{32}\right)\times 0.0821\times 300}{5}\\ & \mathrm{ }=1.23\mathrm{atm},P_{\text{total }}={\mathrm{P}}_{{\mathrm{N}}_2}+{\mathrm{P}}_{{\mathrm{O}}_2}=0.061+1.23 =1.291\approx 1.293\mathrm{atm}\\ & \\ & \\ & \\ & \end{array}$