Question
Calculate the molar mass of a nonvolatile solute if 6.4 g of it dissolved in 100 g water produces a relative lowering in vapour pressure of 0 016 at 300 K.
MHT CET
Chemistry
Solutions
2026
MHT CET 2026 (17 April Shift 2)
MHT CET Chemistry Question (2026) — Solution
Options
- A. 60 gmol ^ -1
- B. 66 gmol ^ -1
- C. 72 gmol ^ -1
- D. 84 gmol ^ -1
Answer
C. 72 gmol ^ -1
Step-by-step solution
The relative lowering in vapour pressure is given by Raoult's law: P P^0 = n_2 n_1 + n_2 For a dilute solution, n_2 n_1, so n_1 + n_2 n_1. The equation simplifies to: P P^0 n_2 n_1 = w_2 M_1 M_2 w_1 Given: Relative lowering in vapour pressure, P P^0 = 0.016 Mass of solute, w_2 = 6.4 g Mass of solvent (water), w_1 = 100 g Molar mass of water, M_1 = 18 g mol ^ -1 Substituting the values into the formula: 0.016 = 6.4 18 M_2 100 M_2 = 6.4 18 0.016 100 M_2 = 115.2 1.6 = 72 g mol ^ -1 Answer: 72 gmol ^ -1
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