States of Matter - Result Question 7
####7. A certain gas takes three times as long to effuse out as helium. Its molecular mass will be :
(a) $27 u$
(b) $36 u$
(c) $64 u$
(d) $9 u$
[2012 M]
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Solution:
- (b) According to Graham’s law of diffusion
$r \propto \sqrt{\frac{1}{d}} \sqrt{\frac{1}{M}} \Rightarrow \frac{r_1}{r_2}=\sqrt{\frac{M_2}{M_1}}$
$\frac{3 r_1}{r_1}=\sqrt{\frac{M_2}{4}} \Rightarrow 9=\frac{M_2}{4}$
$M_2=36 u$
