$350{ \mathrm{cm}}^3\text{ of }{\mathrm{CH}}_4\text{ and }175{ \mathrm{cm}}^3$ of an unknown gas ‘A’ diffused in the same time under similar conditions. The molecular mass of gas A is _____________ .

The rate of diffusion is directly proportional to $\frac{1}{\sqrt{\text{ molecular mass }}}$
 $\begin{array}{l}\therefore \frac{\text{ V olume }}{\text{ Time }}\propto \frac{1}{\sqrt{M}}\\ \Rightarrow \text{ V olume }\propto \frac{1}{\sqrt{M}}\end{array}$
$\begin{array}{l}\frac{{\mathrm{CH}}_4\text{ Volume diffused }}{\text{ Unknown gas volume diffused }}=\sqrt{\frac{\text{ Unknown gas weight }}{{\mathrm{CH}}_4\text{ gas weight }}}\\ \Rightarrow \frac{350}{175}=\sqrt{\frac{\text{ Unknown gas weight }}{16}}\end{array}$

                  = 64​​
Hence, the molecular mass is 64 g/mol.