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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).

1 Answer

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 $\large\frac{50g\;dm^{-3}}{M_2}=\frac{8.6g dm^{-3}}{60g mol^{-1}}$
Hence (A) is the correct answer.
answered Jun 11, 2014 by sreemathi.v

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