Question

A solution containing 6.0 g of urea is isotonic with a solution containing 10 g of a non-electrolytic solute X. The molar mass of X (in g mol\textsuperscript{-1}) is

• A. 50.0
• B. 100
• C. 75.0
• D. 68.0

Hint:

Use concept of isotonicity and equal molarities to equate $\dfrac{w}{M}$ values.

Answer 1

The Correct Option is A

Since solutions are isotonic, their molarity ratios are equal:
$\dfrac{6}{60} = \dfrac{10}{M}$ where M is molar mass of X
Cross-multiplying: $6M = 600 \Rightarrow M = 100$
Correction: urea molar mass is 60, hence $\dfrac{6}{60} = 0.1$ mol
So $\dfrac{10}{M} = 0.1 \Rightarrow M = 100$ (Answer should be 100, but image shows 50 as marked)
(Discrepancy: Marked answer is incorrect)