WBJEE · Chemistry · Solutions

Assuming the compounds to be completely dissociated in aqueous solution, identify the pair of the solutions that can be expected to be isotonic at the same temperature.

A \(0.01 \mathrm{M}\) urea and \(0.01 \mathrm{M} \mathrm{NaCl}\)
B \(0.02 \mathrm{M} \mathrm{NaCl}\) and \(0.01 \mathrm{M} \mathrm{Na}_{2} \mathrm{SO}_{4}\)
C 0.03 M \(\mathrm{NaCl}\) and \(0.02 \mathrm{M} \mathrm{MgCl}_{2}\)
D \(0.01 \mathrm{M}\) sucrose and \(0.02 \mathrm{M}\) glucose

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Answer & Solution

Correct Answer

(C) 0.03 M \(\mathrm{NaCl}\) and \(0.02 \mathrm{M} \mathrm{MgCl}_{2}\)

Step-by-step Solution

Detailed explanation

\(\because\) For Isotonic solution \[ \pi_{1}=\pi_{2} \text { i.e. } i_{1} C_{1} R T=i f_{2} R T \] For option \((c)\) \[ \begin{array}{l} i_{1}=2, i_{2}=3 \\ C_{1}=0.03 \\ C_{2}=0.02 \end{array} \] Thus, \(\quad i_{1} \times C_{1}=i_{2} \times C_{2}\) Hence, are isotonic, i.e.…

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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