Question:

A set of solutions is prepared using 180 g of water as a solvent and 10 g of different nonvolatile solutes A, B and C. The relative lowering of vapour pressure in the presence of these solutes are in the order :
[Given, molar mass of A = 100 g mol\(^{-1}\); B = 200 g mol\(^{-1}\); C = 10,000 g mol\(^{-1}\)]

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Relative lowering of vapour pressure equals the mole fraction of the solute, so a smaller molar mass (more moles) gives a larger lowering.
Updated On: Nov 5, 2026
  • A > C > B
  • C > B > A
  • A > B > C
  • B > C > A
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The Correct Option is C

Solution and Explanation

Relative lowering of vapour pressure \(=\frac{\Delta P}{P^0} = x_{solute} = \frac{n_B}{n_A + n_B}\).
Moles of water \(= \frac{180}{18} = 10\).
Moles of A \(= \frac{10}{100} = 0.1\); B \(= \frac{10}{200} = 0.05\); C \(= \frac{10}{10000} = 0.001\).
\(x_A = \frac{0.1}{10.1} \approx 0.0099\); \(x_B = \frac{0.05}{10.05} \approx 0.00498\); \(x_C = \frac{0.001}{10.001} \approx 0.0001\).
So the order is A > B > C.
Answer: (3) A > B > C
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