Van der Waals constant b of helium is 24 mL mol^-I. Find molecular diameter of helium.

b = 4 x volume occupied by molecules of 1 mole of a gas,
$$\begin{gathered} \mathrm{or} \mathrm{b}=4{\mathrm{N}}_{\mathrm{a}}\left(\frac{4}{3}{\mathrm{\pi r}}^3\right) \\ \mathrm{or} \mathrm{r}={\left(\frac{3\mathrm{b}}{16{\mathrm{N}}_{\mathrm{A}}\mathrm{\pi }}\right)}^{1/3} \end{gathered}$$
$\begin{array}{l}={\left[\frac{3\times 24}{16\times 6.023\times {10}^{23}\times 3.14}\right]}^{1/3}\mathrm{cm}\\ =1.355\times {10}^{-8}\mathrm{cm}\end{array}$
Now molecular diameter, d = 2r
$\approx$ 2.67 x 10^-8 cm