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 kg\text{ }mo{{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{1000{{K}_{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}=256\,gmo{{l}^{-1}} $
But atomic mass of sulphur
$ =32\,g\,mo{{l}^{-1}} $
If molecular formula is $ {{S}_{x}} $ then, $ 32x=256 $
$ x=8 $
Hence, sulphur exists as 83 in benzene.