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Question

# At 20oC, the vapour pressure of 0.1 M solution of urea is 0.0311 mm less than that of water and the vapour pressure of 0.1 M solution of KCl is 0.0574 mm less than that of water. The apparent degree of dissociation of KCl at this dilution is :

A
92.1%
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B
84.6%
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C
68.4%
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D
54.1%
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Solution

## The correct option is A 84.6%The Vant Hoff factor is the property of a solute. So it is directly proportional to any colligative property of solute (like vapour pressure).So, vapour pressure ∝ Vant Hoff factor (i)⟹vapourpressure(urea)vapourpressure(KCl)=iureaiKClBut i(urea)=1⟹vapourpressure(KCl)vapourpressure(urea)=iKCl=0.05740.0311 ...(i)KCl⇋K++Cl− 1 0 01−α α αFor dissociation, we have:α=i−1n−1Here, n=2⟹α=i−12−1⟹i=1+αFrom (i), we have:0.05740.0311=1+α⟹α=0.846So, %α=α×100=84.6%Hence, the correct option is (B).

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