Question

$2.56\, g$ of sulphur in $100\, g$ of $CS _{2}$ has depression in freezing point of $0.010^{\circ} C$.
Atomicity of sulphur in $CS _{2}$ is (Given, $K_{f}=0.1^{\circ}$ molal $^{-1}$ )

• A. 2
• B. 4
• C. 6
• D. 8

Answer 1

Answer: (D)

Let atomicity of sulphur is $n$, then it exists as $S _{n}$.
$\therefore $ Molar mass, $m=32 \,n$
Now from,
$\Delta T_{f} =\frac{1000 \,w \,K_{f}}{m\, w}$
$0.010 =\frac{1000 \times 2.56 \times 0.1}{m \times 100}$
or $m=\frac{1000 \times 2.56 \times 0.1}{0.010 \times 100}=256$
or $32 n=256$
or, $n=\frac{256}{32}=8$