One mole of ${\mathrm{N}}_2{\mathrm{O}}_4$at 300K is kept in a closed vessel at 1 atm. It is heated to 600K when 20% by mass of ${\mathrm{N}}_2{\mathrm{O}}_4$ (g) decomposes to ${\mathrm{NO}}_2\left(\mathrm{g}\right)$.  The resultant equilibrium pressure is 

$\begin{array}{l}{\mathrm{N}}_2{\mathrm{O}}_4\rightleftharpoons 2{\mathrm{NO}}_2\\ 1 0\\ 1-\mathrm{x} 2\mathrm{x}\\ 1-0.2 2\times 0.2\\ 0.8 0.4\end{array}$

$\begin{array}{l}{\mathrm{P}}_{\mathrm{T}}=0.8+0.4=1.2\\ \frac{{\mathrm{P}}_1\mathrm{V}={\mathrm{n}}_1{\mathrm{RT}}_1}{{\mathrm{P}}_2\mathrm{V}={\mathrm{n}}_2{\mathrm{RT}}_2}\\ \frac{{\mathrm{P}}_1}{{\mathrm{P}}_2}=\frac{{\mathrm{n}}_1{\mathrm{T}}_1}{{\mathrm{n}}_2{\mathrm{T}}_2}\\ \frac{1}{{\mathrm{P}}_2}=\frac{1\times 300}{1.2\times 600}\\ {\mathrm{P}}_2=2.4\mathrm{atm}\end{array}$