Question

The rate of effusion of helium is five times that of gas under identical conditions. Its molecular mass will be : [Atomic mass of He = 4u]

Answer 1

according to Graham's law of effusion,

$\frac{r_1}{r_2}=\sqrt{\frac{M_2}{M_1}}$

where, r denotes rate of effusion and M denotes molar mass/atomic mass of gases.

given, $r_{He}=5r_{X}$

atomic mass of He = 4u

let atomic mass of unknown gas is X.

so, $\frac{r_{He}}{r_X}=\sqrt{\frac{X}{4}}$

or, 5 = √{X/4}

or, 25 = X/4

or, X = 100u

hence, molecular mass of unknown gas is 100u.