A solution of an unknown substance X is perfectly isotonic with a solution of urea (molar mass = ) a — Solutions and Colligative Properties Chemistry Question
Question
A $5.25\% \text{ (w/v)}$ solution of an unknown substance X is perfectly isotonic with a $1.5\% \text{ (w/v)}$ solution of urea (molar mass = $60\text{ g mol}^{-1}$) at the same temperature. What is the theoretical molar mass of substance X?
Answer: C
💡 Solution & Explanation
Isotonic solutions have equal effective concentrations ($i_1 C_1 = i_2 C_2$). Assuming X is a non-electrolyte ($i=1$), $\frac{5.25}{M_X} = \frac{1.5}{60} \implies M_X = \frac{5.25 \times 60}{1.5} = 210.0\text{ g mol}^{-1}$.
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