Question

A $5\%$ solution of cane sugar is isotonic with $0.877\%$ solution of $X$. Calculate the molecular mass of $X$, if the molecular mass of cane sugar is $342$ g/mol.

• A. $60$ g/mol
• B. $120$ g/mol
• C. $78$ g/mol
• D. $30$ g/mol

Answer 1

The correct option is A $60$ g/mol

Isotonic solutions have same osmotic pressure.
$⟹{\pi }_1={\pi }_2$
Let $M_X$ be the molar mass of $X$.
Then,
$⟹\frac{5}{342}\times \frac{1000}{100}=\frac{0.877}{M_X}\times \frac{1000}{100}$
$M_X=\frac{0.877\times 342}{5}$ = $59.98$ g/mol

Option A is correct.