A cylinder contains 20 kg of $N_2$ gas $\left(M=28{\mathrm{kgk}}^{-1}{\mathrm{mol}}^{-1}\right)$at a pressure of 5 atm. The mass of hydrogen $\left(M=2kg{k}^{-1},mo{l}^{-1}\right)$ at pressure of 3 atm contained in the same cylinder at same temperature

$\frac{V}{T}=\frac{nR}{P}$

n is number of moles; R is universal gas constant; P is pressure,

volume V  and temperature T for  both cases in same 

Hence

$$\begin{gathered} \frac{n_1}{p_1}=\frac{n_2}{p_2} \\ here n=\frac{mass}{molecular weight} \\ \begin{array}{c}\frac{m_1}{P_1{M}_1}=\frac{m_2}{P_2{M}_2} ;substitute given values\\ {\mathrm{m}}_2=\frac{\left(3\right)\left(2\right)}{\left(5\right)\left(28\right)}\times 20=0.86\mathrm{kg}\end{array} \end{gathered}$$