GATE 2021 CY – Question 61

Physical Chemistry · Solutions: Ideal and non-ideal solutions, Raoult’s Law and Henry’s Law. · 2 marks · Numerical answer

Toluene (molecular weight 92) has vapour pressure 0.13 atm at 25 °C. Dissolving 6 g hydrocarbon in 92 g toluene lowers it to 0.12 atm. The hydrocarbon molar mass is ______ g/mol (nearest integer).

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Correct answer: 71 to 73

Explanation

There is 92/92=1 mol toluene. For a nonvolatile solute, Raoult’s law gives xsolvent=0.12/0.13=12/13.

Therefore 1/(1+nsolute)=12/13, giving nsolute=1/12 mol. The molar mass is 6/(1/12)=**72 g/mol**. Using the exact mole fraction avoids the dilute approximation.