JEE Main 2022 · 27 July · morning · Single correct · +4 / −1

Boiling point of a 2 % aqueous solution of a non-volatile solute A is equal to the boiling point of 8 % aqueous solution of a non-volatile solute B. The relation between molecular weights of A and B is

  1. (a) M_A=4 M_B
  2. (b) M_B=4 M_A
  3. (c) M_A=8 M_B
  4. (d) M_B=8 M_A
Answer and solution

Answer (b) M_B=4 M_A

Solution

Chemistry ma’am · 3 steps

Same boiling point means same molality, so 2 g of A must hold as many moles as 8 g of B.

  1. Same elevation. Both solvents are water, so K_b is the same. Since Δ T_b = K_b m, an equal boiling point means the two molalities are equal.
  2. Moles per solvent. Take 100 g of each solution: 2 g of A and 8 g of B. In the standard treatment the water mass (98 g and 92 g) is taken as about the same, so equal molality means equal moles of solute.

    2/M_A = 8/M_B ⇒ M_B = 4M_A

  3. So. B needs 4 times the mass to give the same number of moles, so its molar mass is 4 times larger: M_B = 4M_A, option (b).

The idea to keep. Equal colligative effect in the same solvent means equal moles of particles. If one solute needs more grams to get there, it has the heavier molecules.

Answer from the NTA final answer key. Solution by PrepWiser.

Study Solutions & Colligative Properties

Asked about 150 times in JEE Main since 2020. This question is from 2022.