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MCQEasyJEE 2023Raoult's Law & Vapour Pressure

JEE Chemistry 2023 Question with Solution

The vapor pressure vs. temperature curve for a solution-solvent system is shown below. The boiling point of the solvent is:

Graph of vapour pressure versus temperature in degree Celsius showing two rising curves, a horizontal line at 1 atm, and marked temperatures 80, 81, 82, and 83 degrees Celsius.
  • A

    81∘C81^\circ \text{C}

  • B

    82∘C82^\circ \text{C}

  • C

    83∘C83^\circ \text{C}

  • D

    84∘C84^\circ \text{C}

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: A vapor pressure versus temperature graph for a solution-solvent system is provided.

Find: The boiling point of the solvent.

The boiling point is defined as the temperature at which the vapor pressure becomes equal to 1 atm1 \, \text{atm}.

From the graph, the solvent curve intersects the horizontal line at 1 atm1 \, \text{atm} at 82∘C82^\circ \text{C}.

Therefore, the boiling point of the solvent is 82∘C82^\circ \text{C}. The correct option is B.

Common mistakes

  • Confusing the solution curve with the solvent curve. This is wrong because the question asks only for the boiling point of the solvent. Identify the solvent curve correctly before reading the temperature at 1 atm1 \, \text{atm}.

  • Reading the temperature at some vapor pressure other than 1 atm1 \, \text{atm}. This is wrong because boiling point is specifically defined at atmospheric pressure. Always locate the intersection with the 1 atm1 \, \text{atm} line first.

  • Assuming the lower or higher temperature directly without checking the graph intersection. This is wrong because the answer must come from the plotted curve, not from a general idea about boiling point elevation. Read the exact temperature marked on the graph.

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