A certain gas takes three times as long to effuse out as helium. Its molecular mass will be:

36 u

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64 u

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9 u

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27 u

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Solution

The correct option is B $36 u$
A certain gas takes three times as long to effuse out as helium.
The rate of effusion is inversely proportional to the molecular mass.
$\frac{r_{1}}{r_{2}} = \sqrt{\frac{M_{w_{2}}}{M_{w_{1}}}}$
The rate of effusion is the ratio of the volume effused to the time taken
$\frac{V_{1}}{t_{1}} \times \frac{t_{2}}{V_{2}} = \sqrt{\frac{M_{w_{2}}}{M_{w_{1}}}}$
Volume is same, so $\frac{3}{1} = \sqrt{\frac{M_{w_{2}}}{4}}$
$9 = \frac{M_{w_{2}}}{4}$
$∴ M_{w_{2}} = 36$

The molecular mass of the gas is $36 u$ . So, the correct option is $A$

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