A 0.1 molal aqueous solution of a weak acid is 30% ionized. If Kf for water is 1.86oC/m, the freezing point of the solution will be:
A
−0.24oC
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B
−0.18oC
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C
−0.54oC
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D
−0.36oC
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Solution
The correct option is A−0.24oC ΔTf=i∗m∗Kfwhere,i=VantHoff′sFactorKf=Freezingpointdepressionconstantm=Molalityofsolution.i=1+(n−1)∗αwhereα=Degreeofdissociation.So,i=1+(2−1)∗0.3=1.3ΔTf=1.3∗1.86∗0.1=0.24Tf=T0−ΔTf=0−0.24=−0.240C