A non-volatile solute, 'A' tetramerises in water to the extent of 80%. 2.5g of 'A' in 100 g of water, lower the freezing point by 0.3oC. The molar mass of A in (g/mol) is: (Kf for water = 1.86 Kkgmol−1)
A
62
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B
221
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C
155
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D
354
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Solution
The correct option is A 62 ΔTf=i×Kf×m, α=0.8 4A⇌A4
i=4−4α+α4 Van 't Hoff factor (i) = 0.4 Given that ΔT=0.3K,m=25M (M = Molar mass of A), 0.3=0.4×1.86×25MM=62g/mol