Q16 / 120 · JEE Main 2007 · Chemistry

A $5.25 \%$ solution of a substance is isotonic with a $1.5 \%$ solution of urea $\left(\right.$ molar mass $=60 \mathrm{~g} \mathrm{~mol}^{-1}$ ) in the same solvent. If the densities of both the solutions are assumed to be equal to $1.0 \mathrm{~g} \mathrm{~cm}^{-3}$, molar mass of the substance will be

  • A
    $90.0 \mathrm{~g} \mathrm{~mol}^{-1}$
  • B
    $115.0 \mathrm{~g} \mathrm{~mol}^{-1}$
  • C
    $105.0 \mathrm{~g} \mathrm{~mol}^{-1}$
  • Correct answer
    D
    $210.0 \mathrm{~g} \mathrm{~mol}^{-1}$

Solution

Solutions with the same osmotic pressure are isotonic Let the molar mass of the substance be $M$ $\pi_1=\mathrm{C}_1 \mathrm{RT}=\mathrm{C}_2 \mathrm{RT}=\pi_2$ So, $\mathrm{C}_1=\mathrm{C}_2$ As density of the solutions are same So $\frac{5.25}{M}=\frac{15}{60}$ $M=\frac{5.25 \times 60}{1.5}=210$ Hence (D) is correct

Hence, the correct answer is option: 'D'.

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