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MCQEasyJEE 2025Electrolytic Conductance & Kohlrausch's Law

JEE Chemistry 2025 Question with Solution

Given below are two statements:

Statement I: Mohr's salt is composed of only three types of ions—ferrous, ammonium, and sulphate.

Statement II: If the molar conductance at infinite dilution of ferrous, ammonium, and sulphate ions are x1x_1, x2x_2, and x3x_3 S  cm2 mol−1S \; cm^2 \, mol^{-1}, respectively, then the molar conductance for Mohr's salt solution at infinite dilution would be given by x1+x2+2x3x_1 + x_2 + 2x_3.

  • A

    Both statements I and Statement II are false

  • B

    Statement I is false but Statement II is true

  • C

    Statement I is true but Statement II is false

  • D

    Both statements I and Statement II are true

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: Two statements about Mohr's salt and its molar conductance at infinite dilution are to be checked.

Find: Which statement is true and which statement is false.

Step 1: Analyze Statement I Mohr's salt is ammonium ferrous sulphate. It contains the ion types Fe2+\text{Fe}^{2+}, NH4+\text{NH}_4^+, and SO42−\text{SO}_4^{2-}.

Therefore, Statement I is true.

Step 2: Analyze Statement II At infinite dilution, the molar conductance of an electrolyte is the sum of the molar ionic conductances of the ions produced.

The extracted solution states that the correct expression is

λ∞=x1+x2+x3\lambda_{\infty} = x_1 + x_2 + x_3

and not

x1+x2+2x3x_1 + x_2 + 2x_3

So Statement II is false according to the provided solution working.

Therefore, Statement I is true but Statement II is false. The correct option is C.

Stepwise Evaluation

Given:

  • Ionic conductances at infinite dilution are x1x_1, x2x_2, and x3x_3 for ferrous, ammonium, and sulphate ions, respectively.
  • We must test the truth of Statement I and Statement II.

Find: The correct option.

From the solution, Mohr's salt is described as containing only three types of ions: ferrous, ammonium, and sulphate. Hence Statement I is taken as true.

For Statement II, the provided solution concludes that the sulphate contribution should not be doubled, and therefore the molar conductance at infinite dilution is written as

λ∞=x1+x2+x3\lambda_{\infty} = x_1 + x_2 + x_3

Thus the expression

x1+x2+2x3x_1 + x_2 + 2x_3

is marked incorrect in the source solution.

Conclusion: Based on the solution, Statement I is true and Statement II is false. Hence the correct option is C.

Common mistakes

  • Assuming that an ion's contribution must be multiplied by its charge. Ionic molar conductance is added as given for each ion in the dissociation expression used in the provided solution, not by multiplying directly with charge.

  • Confusing the number of ion types with the number of ions present after dissociation. Statement I speaks about types of ions, so ferrous, ammonium, and sulphate are the relevant categories.

  • Accepting the expression x1+x2+2x3x_1 + x_2 + 2x_3 without checking the solution logic. The provided solution explicitly concludes that the sulphate term should not be doubled and writes λ∞=x1+x2+x3\lambda_{\infty} = x_1 + x_2 + x_3 instead.

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