Here , given that kf = 5.12 K kg mol-1 ( I did not understand the unit you have mentioned ie ., kg mil per inverse but 5.12 K kg mol-1 is the correct value. ) , 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