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Solutions question

2025 · 22 Jan · Shift 1 · Q15
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Solutions question

2025 · 22 Jan · Shift 1 · Q15

JEE MainChemistrySolutionsMCQ+4 / −1
Arrange the following solutions in order of their increasing boiling points. (i) 10−4MNaCl10^{-4} \mathrm{M} \mathrm{NaCl}10−4MNaCl(ii) 10−4M10^{-4} \mathrm{M}10−4M Urea (iii) 10−3MNaCl10^{-3} \mathrm{M} \mathrm{NaCl}10−3MNaCl(iv) 10−2MNaCl10^{-2} \mathrm{M} \mathrm{NaCl}10−2MNaCl
  1. A
    ((( i )<()\lt ()<( ii )<()\lt ()<( iii )<()\lt ()<( iv )))
  2. B
    (ii) <(\lt (<( i )<()\lt ()<( iii )<()\lt ()<( iv )))
  3. C
    (iv) <(\lt (<( iii )<()\lt ()<( i )<()\lt ()<( ii )))
  4. D
    (ii) <\lt<(i) ≡\equiv≡(iii) <\lt< (iv)
View written solutionFree

Correct answer: B

  1. Use boiling point elevation relation

For dilute solutions, ΔTb=iKbm\Delta T_b = i K_b mΔTb​=iKb​m where:

  • iii = van’t Hoff factor
  • KbK_bKb​ = ebullioscopic constant of solvent
  • mmm = molality

Since all are very dilute aqueous solutions, we compare i×i \timesi× concentration directly.

  1. Find effective particle concentration for each solution

  • (i) 10−4 M NaCl10^{-4}\,\text{M NaCl}10−4M NaCl

    NaCl dissociates as: NaCl→Na++Cl−\text{NaCl} \rightarrow \text{Na}^+ + \text{Cl}^-NaCl→Na++Cl− So, ideally i≈2i \approx 2i≈2. Hence relative boiling point elevation: iC=2×10−4=2×10−4iC = 2 \times 10^{-4} = 2 \times 10^{-4}iC=2×10−4=2×10−4

  • (ii) 10−4 M urea10^{-4}\,\text{M urea}10−4M urea

    Urea is a non-electrolyte, so: i=1i=1i=1 Hence, iC=1×10−4=10−4iC = 1 \times 10^{-4} = 10^{-4}iC=1×10−4=10−4

  • (iii) 10−3 M NaCl10^{-3}\,\text{M NaCl}10−3M NaCl iC=2×10−3iC = 2 \times 10^{-3}iC=2×10−3

  • (iv) 10−2 M NaCl10^{-2}\,\text{M NaCl}10−2M NaCl iC=2×10−2iC = 2 \times 10^{-2}iC=2×10−2

  1. Compare boiling point elevations

Greater ΔTb\Delta T_bΔTb​ means greater boiling point.

So the increasing order is: 10−4<2×10−4<2×10−3<2×10−210^{-4} < 2\times 10^{-4} < 2\times 10^{-3} < 2\times 10^{-2}10−4<2×10−4<2×10−3<2×10−2 which corresponds to: (ii)<(i)<(iii)<(iv)(ii) < (i) < (iii) < (iv)(ii)<(i)<(iii)<(iv)

  1. Match with the options

This matches Option B.

  1. Comparison with stored correct answer

Stored correct answer: B

Our derived answer: B

So, they agree.

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