50 mL of a gas ‘A’ diffuse through a membrane in the same time as for the diffusion of 40mL of gas B under identical conditions of pressure and temperature. If the molecular mass of A is 64, that of B would be;

The rate of diffusion of for two gases A and B with the molecular mass as ${\mathrm{M}}_{\mathrm{A}} \mathrm{and} {\mathrm{M}}_{\mathrm{B}}$ is

$\frac{{\mathrm{r}}_{\mathrm{A}}}{{\mathrm{r}}_{\mathrm{B}}}=\sqrt{\frac{M_B}{M_A}}$

At same period of time A and B produce different volumes.

${\mathrm{V}}_{\mathrm{A}}=50 \mathrm{ml}, {\mathrm{V}}_{\mathrm{B}}=40 \mathrm{ml}$

$then \frac{{\mathrm{V}}_{\mathrm{A}}}{{\mathrm{V}}_{\mathrm{B}}}=\sqrt{\frac{M_B}{M_A}}$

$\Rightarrow \frac{50}{40}=\sqrt{\frac{M_B}{{ }_{64}}}$

$\frac{5}{4}=\sqrt{\frac{M_B}{8_2}}$

$\sqrt{{\mathrm{M}}_{\mathrm{B}}}=10 and M_B= 100$