Q. On adding 1 g arsenic to 80 g benzene, the freezing point of benzene is lowered by $0.19℃\ .$ The formula of arsenic is
A) As
B) $As_{2}$
C) $As_{3}$
D) $As_{4}$
Explanation
(d)
$\ \ \ \ \ \ \ \mathrm{\Delta}T_{f} = \frac{1000\ k_{f}w}{mW}$
$\ \ \ \ \ \mathrm{\Delta}T_{f} = 0.19℃$; $k_{f} =$5.08 kg $mol^{- 1}$, w=1g, W=80g
M=$\frac{1000k_{f}w}{\mathrm{\Delta}T_{f}W}$
=$\frac{1000 \times 5.08 \times 1}{0.19 \times 80}$ = 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 ${As}_{4}.$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.