Q. A 5.25% solution of a substance is isotonic with a 1.5% solution of urea (molar mass =60 g$mol^{- 1}$) in the same solvent. If the densities of both the solutions are assumed to be equal to 1.0 g $cm^{- 3},\ molar\$mass of the substance will be
A) 90.0 g $mol^{- 1}$
B) 115.0 g $mol^{- 1}$
C) 105.0 g $mol^{- 1}$
D) 210.0 g $mol^{- 1}$
Explanation
(d)
Solution is isotonic.
$\Rightarrow$ $C_{1}RT = C_{2}RT\$
$C_{1} = C_{2}$
Density of both the solutions are assumed to be equal to
1.0 g ${cm}^{- 3}$.
5.25% solution of a substance means 100g solution contains
5.25 g solute and 1000g solution contain 52.5g solute.
Hence, $\frac{52.5}{M} = \frac{15}{60},$
M= molecular mass of the substance
M= $\frac{52.5 \times 60}{15}$=210
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.