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MCQEasyJEE 2025Colligative Properties

JEE Chemistry 2025 Question with Solution

Arrange the following solutions in order of their increasing boiling points.

(i) 10−410^{-4} M NaCl (ii) 10−410^{-4} M Urea

(iii) 10−310^{-3} M NaCl (iv) 10−210^{-2} M NaCl

  • A

    ((ii) < (i) < (iii) < (iv))

  • B

    ((ii) < (i) = (iii) < (iv))

  • C

    ((i) < (ii) < (iii) < (iv))

  • D

    ((iv) < (iii) < (i) < (ii))

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: The solutions are (i) 10−410^{-4} M NaCl, (ii) 10−410^{-4} M urea, (iii) 10−310^{-3} M NaCl, and (iv) 10−210^{-2} M NaCl.

Find: The order of increasing boiling points.

Boiling point elevation is a colligative property and depends on the number of solute particles present. The relation is

ΔTb=iKbm\Delta T_b = iK_b m

where ii is the van't Hoff factor.

For the given solutions:

  • (i) NaCl dissociates into 22 ions, so i=2i=2.
  • (ii) Urea does not dissociate, so i=1i=1.
  • (iii) NaCl has i=2i=2.
  • (iv) NaCl has i=2i=2.

For very dilute solutions, compare i×i \times concentration:

(i):2×10−4(ii):1×10−4(iii):2×10−3(iv):2×10−2\begin{aligned} (i)&: 2 \times 10^{-4} \\ (ii)&: 1 \times 10^{-4} \\ (iii)&: 2 \times 10^{-3} \\ (iv)&: 2 \times 10^{-2} \end{aligned}

As boiling point increases with increasing value of i×i \times concentration, the order is:

(ii)<(i)<(iii)<(iv)(ii) < (i) < (iii) < (iv)

Therefore, the correct option is A.

Common mistakes

  • Assuming only concentration matters. This is wrong because boiling point elevation depends on the total number of solute particles. Include the van't Hoff factor ii along with concentration.

  • Treating NaCl and urea as equivalent at the same concentration. This is wrong because NaCl dissociates into ions while urea does not. Compare i×i \times concentration, not concentration alone.

  • Using the higher concentration order correctly but placing (i) and (ii) together. This is wrong because even at the same 10−410^{-4} M concentration, NaCl gives more particles than urea. Hence (ii) must come before (i).

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