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Question

15.0 g of an unknown molecular material was dissolved in 450 g of water. The resulting solution was found to freeze at 0.34oC. What is the molar mass of this material? (Kf for water=1.86 Kkg mol1)

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Solution

(b) Given mass of solute =wb=15.0g
Molar mass of solute =Mb=?
Mass of water =wa=450g
Freezing point of water =0^AoC=273K
Freezing point of solution =0.34^AoK
=(0.34+273)K
Depression in freezing point ="Tf=273(0.34+273)=0.34K
Kf for water = 1.86 K Kg mol1
Since, ΔTf=Kf×m=Kfwb×1000Mb×wa
Mb=Kf×wb×1000waΔTf
Mb=1.86×15×1000450×0.34
=182.35g/mol

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