N₂ is obtained by the decomposition of benzene diazonium chloride at constant temperature. It is a first - order reaction the various volumes obtained at different time intervals are as    

Time (min)	0	20	50	70	$\infty$
N2(mL)	0	10	25	32	162

The rate constant of the reaction is

Decomposition of benzene diazonium chloride occur the following manner.
${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{N}=\mathrm{NCl}⟶{\mathrm{C}}_6{\mathrm{H}}_5\mathrm{Cl}+{\mathrm{N}}_2$
The volume of nitrogen obtained shows the extent of decomposition of benzene diazonium chloride so
   $\begin{array}{r}\mathrm{a}\propto {\mathrm{V}}_{\mathrm{\infty }}=162\mathrm{mL}\\ (\mathrm{a}-\mathrm{x})\propto \left({\mathrm{V}}_{\mathrm{\infty }}-{\mathrm{V}}_{\mathrm{t}}\right)\mathrm{mL}\end{array}$
For first order reaction, $\mathrm{k}=\frac{2.303}{\mathrm{t}}{\log }_{10}\frac{{\mathrm{V}}_{\mathrm{\infty }}-{\mathrm{V}}_0}{{\mathrm{V}}_{\mathrm{\infty }}-{\mathrm{V}}_{\mathrm{t}}}$
or                                   $\mathrm{k}=\frac{2.303}{\mathrm{t}}{\log }_{10}\frac{{\mathrm{V}}_{\mathrm{\infty }}}{{\mathrm{V}}_{\mathrm{\infty }}-{\mathrm{V}}_{\mathrm{l}}}$                    $\left(\because {\mathrm{V}}_0=0\right)$
At 20 min                     $\mathrm{k}=\frac{2.303}{20}{\log }_{10}\frac{162}{162-10}$
                                     $\begin{array}{r} =\frac{2.303}{20}{\log }_{10}\frac{162}{152}\\ =3.18\times {10}^{-3}{\min }^{-1}\\ \approx 3.2\times {10}^{-3}{\min }^{-1}\end{array}$