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Solutions · NEET Preparation Chemistry

The vapor pressure of pure benzene at a certain temperature is 0.850 text{ bar}. A non-volatile, non-electrolyte solid weighing 0.5 text{ g} is added…

Practice question from Solutions (NEET Preparation Chemistry), with the correct answer and a step-by-step solution.

hard · MCQ · Q-65BC48

The vapor pressure of pure benzene at a certain temperature is 0.850 bar0.850 \text{ bar}. A non-volatile, non-electrolyte solid weighing 0.5 g0.5 \text{ g} is added to 39.0 g39.0 \text{ g} of benzene (molar mass 78 g/mol78 \text{ g/mol}). The vapor pressure of the solution is 0.845 bar0.845 \text{ bar}. What is the molar mass of the solid substance?

  1. A.58 \text{ g/mol}
  2. B.170 \text{ g/mol}Correct answer
  3. C.34 \text{ g/mol}
  4. D.68 \text{ g/mol}

Solution

Using relative lowering of vapor pressure: (P0−Ps)/P0=n2/(n1+n2)≈n2/n1(P^0 - P_s) / P^0 = n_2 / (n_1 + n_2) \approx n_2 / n_1. (0.850−0.845)/0.850=(0.5/M)/(39/78)  ⟹  0.005/0.850=(0.5/M)/0.5=1/M(0.850 - 0.845) / 0.850 = (0.5 / M) / (39 / 78) \implies 0.005 / 0.850 = (0.5 / M) / 0.5 = 1 / M. Thus, M=0.850/0.005=170 g/molM = 0.850 / 0.005 = 170 \text{ g/mol}.

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