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NVAEasyJEE 2023Colligative Properties

JEE Chemistry 2023 Question with Solution

The number of pairs of the solution having the same value of osmotic pressure from the following is:

A. 0.500 M0.500 \, \text{M} C2H5OH\mathrm{C_2H_5OH} (aq)\text{(aq)} and 0.25 M0.25 \, \text{M} KBr\mathrm{KBr} (aq)\text{(aq)}

B. 0.100 M0.100 \, \text{M} K4[Fe(CN)6]\mathrm{K_4[Fe(CN)_6]} (aq)\text{(aq)} and 0.100 M0.100 \, \text{M} FeSO4(NH4)2SO4\mathrm{FeSO_4(NH_4)_2SO_4} (aq)\text{(aq)}

C. 0.05 M0.05 \, \text{M} K4[Fe(CN)6]\mathrm{K_4[Fe(CN)_6]} (aq)\text{(aq)} and 0.25 M0.25 \, \text{M} NaCl\mathrm{NaCl} (aq)\text{(aq)}

D. 0.15 M0.15 \, \text{M} NaCl\mathrm{NaCl} (aq)\text{(aq)} and 0.1 M0.1 \, \text{M} BaCl2\mathrm{BaCl_2} (aq)\text{(aq)}

E. 0.02 M0.02 \, \text{M} KCl\mathrm{KCl} MgCl2.6H2O\mathrm{MgCl_2.6H_2O} (aq)\text{(aq)} and 0.05 M0.05 \, \text{M} KCl\mathrm{KCl} (aq)\text{(aq)}

Answer

Correct answer:4

Step-by-step solution

Standard Method

Given: Several pairs of aqueous solutions are listed.

Find: The number of pairs having the same osmotic pressure.

Osmotic pressure depends on the total concentration of dissolved particles. For dilute solutions,

π=iCRT\pi = iCRT

So, under the same temperature, we compare the value of iCiC for each solution.

From the solution: Pair A, B, D, and E have the same osmotic pressures, while Pair C does not.

Therefore, the number of pairs with the same osmotic pressure is 44.

Using ion concentration idea

Given: Osmotic pressure is determined by the total ion concentration in solution.

Find: How many listed pairs have equal osmotic pressure.

The extracted explanation states that osmotic pressure is directly proportional to the total concentration of dissolved particles and asks us to compare the number of ions produced on dissociation.

It explicitly concludes that A, B, D, and E have the same osmotic pressure.

Hence, the required count is 44.

Common mistakes

  • Treating osmotic pressure as depending only on molarity. This is wrong because dissolved particle count also matters through the van't Hoff factor ii. Compare iCiC, not only CC.

  • Ignoring dissociation of ionic solutes such as KBr\mathrm{KBr}, NaCl\mathrm{NaCl}, or BaCl2\mathrm{BaCl_2}. This underestimates the number of solute particles. Count the ions produced in solution.

  • Comparing formulas qualitatively without checking the total particles numerically. Different compounds can still give the same osmotic pressure if their effective particle concentrations are equal. Compute or infer the effective ion concentration for each pair.

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