1 g of non-electrolyte solute dissolved in 50g of benzene lowered the freezing point of benzene by 0.40K. The freezing point depression constant of benzene is 5.12 K kg/mol. Find the molar mass of the solute.

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Solution

Here , given that kf = 5.12 K kg mol-1
W1( mass of solvent) = 50 g ,
W2( mass of solute ) = 1 g ,
∆Tf = 0.4 K.
We need to find M2 , the molar mass of solute.
From the relation , we have
∆Tf = kf x W2 x 1000/ W1 x M2
So ,
M2 = kf x W2 x 1000/W1 x ∆Tf
= 5.12 K kg mol-1 x 1 g x 1000/ 50 g x 0.4 K
=5.12 x 20/0.4
=256 g

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