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Question

1.02g of urea when dissolved in 98.5g of certain solvent decreases its freezing point by 0.211K.1.60g of an unknown compound when dissolved in 86.0g of certain solvent depresses the freezing point by 0.34K. Calculate the molar mass of the unknown compound. (Urea NH2CONH2,N=14,C=12,O=16,H=1)

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Solution

Amount of urea = 1.02g
no. of mole = 1.02/60
Molar mass of urea (NH2CONH2)=60
Molality = 1.0260×100098.5=0.17259
ΔTf=kfm0.211=kf×0.1759 ...(i)
Let molar mass of unknown compound = M
ΔTf=kfm
ΔTf=kd×1.60M.100080.6
0.34=kf×19.85M ...(ii)
divide equation (i) by (ii)
0.2110.34=0.175919.85×M
M=0.211×19.850.34×17.59=70gm

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