Q. The vapour pressure of water at $20℃$ is 17.54 mm. When 20 g of a non-ionic, substance is dissolved in 100 g of water, the vapour pressure is lowered by 0.30 mm. What is the molecular mass of the substance?
A) 200.8
B) 206.88
C) 210.5
D) 215.2
Explanation
(c)
$\frac{p^{0} - p_{s}}{p^{0}} = \frac{w}{m} \times \frac{M}{w}$
$\frac{0.30\ mm}{17.54\ mm} = \frac{20}{m} \times \frac{18}{100}\ \Rightarrow m = \frac{20 \times 18 \times 17.54}{0.30 \times 100} = 210.48$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.