van der Waals constant b of helium is 24 mL mol-1 . Find molecular diameter of helium.

b = 4 x volume occupied by molecules of 1 mol of a gas,or     b=4NA43πr3or     r=3b16NAπ1/3      =3×2416×6.023×1023×3.141/3cm=1.355×10−8cmNow, molecular diameter, d = 2r= 2.67 x 10-8 cm