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# A solution containing 8.6g per $dm^3$ of urea (molar mass = 60g $mol^{–1}$) was found to be isotonic with a 5 percent solution of an organic non-volatile solute. Calculate the molar mass of the latter (solute).

According to the van’t Hoff theory, isotonic solutions have the same osmotic pressure at the same temperature and the same molar concentration
Let $M_2$ be the molar mass of the unknown solute
Molar concentration of urea solution =$\large\frac{50gdm^{-3}}{M_2}$
Since both the solutions are isotonic, hence, by definition, their molar concentrations are equal.
Thus $\large\frac{50g\;dm^{-3}}{M_2}=\frac{8.6g dm^{-3}}{60g mol^{-1}}$
$M_2=348.8gmol^{-1}$
Hence (A) is the correct answer.