Question

On adding $1\, g$ arsenic to $80\, g$ benzene, the freezing point of benzene is lowered by $0.19^{\circ} C$. The formula of arsenic is $\left(K_{f}=5,08\, K\, kg\, mol ^{-1}\right)$.

• A. $As$
• B. $As _{2}$
• C. $As _{3}$
• D. $As _{4}$

Answer 1

Answer: (D)

$\Delta T_{f}=\frac{1000 K_{f} w}{m W}$
$\Delta T_{f}=0.19^{\circ} C ; K_{f}=5.08\, K\, kg\, mol ^{-1},$
$w=1\, g ; W=80\, g$
$m=\frac{1000 K_{f} w}{\Delta T_{f} W}=\frac{1000 \times 5.08 \times 1}{0.19 \times 80}=334.21$
Atomic weight of $As =74.92$
Hence, number of atoms $=\frac{334.21}{74.92} \approx 4$
Hence, the formula of arsenic is $As _{4}$.