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Question

# To 500 cm3 of water, 3.0×10−3 kg of acetic acid is added. If 23% of acetic acid is dissociated, what will be the depression in freezing point. If kf and density of water are 1.86 K kg−1 and 0.997 g cm−3 respectively.

A
0.186 K
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B
0.228 K
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C
0.372 K
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D
0.556 K
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Solution

## The correct option is B 0.228 KNumber of moles of acetic acid= 3×10−3kg60gmol−1=3g60gmol−1=0.05Mass of water= m1=500cm3×0.997gcm−3=0.4985kgMolality of acetic acid= m=n2m1=0.05mol0.4985kg=0.1003molkg−1Dissociation of CH3COOH: CH3COOH⇌CH3COO⊝+H⊕t=0 1 0 0eqm 1−α α αTotal number of moles= i=1−α+α+α=1+αas, ΔTf=iKfmΔT=(1+α)Kfm=(1+0.23)(1.86 K Kg mol−1)(0.1003 mol Kg−1)=0.228KHence, the correct option is B

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