Question

25.6 g of sulphur in 100 g benzene shows depression in freezing point of ${5.12}^{o}C$. $K_f$ for benzene is ${5.12}^{o}Ckgmo{l}^{-1}$ . The molecular formula of sulphur in benzene is:

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

Answer 1

The correct option is C $S_8$

$\Delta T_f=\frac{1000K_f\times w_2}{M_2\times w_1}=\frac{1000\times 5.12\times 25.6}{M_2\times 100}=5.12$

$M_2=\frac{1000\times 5.12\times 25.6}{5.12\times 100}=256gmo{l}^{-1}$

But atomic mass of sulphur $=32gmo{l}^{-1}$

If molecular formula is $S_x$ then, $32x=256$

$x=8$

Hence, sulphur exists as $S_8$ in benzene.