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Solutions question

2002 · Shift 0 · Q20
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Solutions question

2002 · Shift 0 · Q20

JEE MainChemistrySolutionsMCQ+4 / −1
Freezing point of an aqueous solution is (-0.186)oC. Elevation of boiling point of the same solution is Kb = 0.512 oC, Kf = 1.86 oC, find the increase in boiling point.
  1. A
    0.186 oC
  2. B
    0.0512 oC
  3. C
    0.092 oC
  4. D
    0.2732 oC
View written solutionFree

Correct answer: B

  1. Use colligative property relations

For the same solution, ΔTf=Kfm\Delta T_f = K_f mΔTf​=Kf​m and ΔTb=Kbm\Delta T_b = K_b mΔTb​=Kb​m where mmm is the molality.

  1. Given freezing point depression

The freezing point of the solution is −0.186∘C-0.186^\circ\text{C}−0.186∘C.

Since pure water freezes at 0∘C0^\circ\text{C}0∘C, the depression in freezing point is ΔTf=0−(−0.186)=0.186∘C\Delta T_f = 0 - (-0.186) = 0.186^\circ\text{C}ΔTf​=0−(−0.186)=0.186∘C

  1. Find molality relation

From ΔTf=Kfm\Delta T_f = K_f mΔTf​=Kf​m we get m=ΔTfKf=0.1861.86m = \frac{\Delta T_f}{K_f} = \frac{0.186}{1.86}m=Kf​ΔTf​​=1.860.186​

  1. Calculate boiling point elevation

Now, ΔTb=Kbm=Kb(ΔTfKf)\Delta T_b = K_b m = K_b\left(\frac{\Delta T_f}{K_f}\right)ΔTb​=Kb​m=Kb​(Kf​ΔTf​​)

So, ΔTb=0.512×0.1861.86\Delta T_b = 0.512 \times \frac{0.186}{1.86}ΔTb​=0.512×1.860.186​

Since 0.1861.86=0.1\frac{0.186}{1.86} = 0.11.860.186​=0.1 therefore, ΔTb=0.512×0.1=0.0512∘C\Delta T_b = 0.512 \times 0.1 = 0.0512^\circ\text{C}ΔTb​=0.512×0.1=0.0512∘C

  1. Match with options

ΔTb=0.0512∘C\boxed{\Delta T_b = 0.0512^\circ\text{C}}ΔTb​=0.0512∘C​

So the correct option is B.

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