Question

25.6 g of sulphur in 100 g benzene shows depression in freezing point of $5.12{}^{\circ }$ . Ks for benzene is $5.12{}^{\circ }kgmo{l}^{-1}$ . Molecular formula of sulphur in benzene is

• A. $S_2$
• B. $S_6$
• C. $S_8$
• D. $S_{12}$

Answer 1

Answer: (C)

$\Delta T_f=\frac{1000K_f{w}_2}{M_2\times w_1}=\frac{1000\times 5.12\times 25.6}{M_2\times 100}=5.12$
$M_2=\frac{1000\times 5.12\times 25.6}{5.12\times 100}=256gmo{l}^{-1}$
But atomic mass of sulphur
$=32gmo{l}^{-1}$
If molecular formula is $S_x$ then, $32x=256$
$x=8$
Hence, sulphur exists as 83 in benzene.