Question

Suppose 1 g of lauric acid is added to 5 g of stearic acid. Predict the temperature at which this mixture will begin to melt, and justify your answer.

Now suppose 1 g stearic acid is added to 5 g of lauric acid. Predict the temperature at which this mixture will begin to melt, and justify your answer.

Answer 1

First : 1 g lauric acid in 5 g stearic acid

molality of solution = 1 g lauric acid/200.32 g/mol x 0.005 kg = 1.0 m

normal melting point of stearic acid = 69 oC

Kf for stearic acid = 4.5 oC/m

Van't hoff factor = 1

Using,

dTf = iKfm

      = 1 x 4.5 x 1.0

      = 4.5 oC

melting point of mixture = 69 - 4.5 = 64.5 oC

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second : 1 g stearic acid in 5 g lauric acid

molality of solution = 1 g stearic acid/284.48 g/mol x 0.005 kg = 0.0703 m

normal melting point of stearic acid = 43.2 oC

Van't hoff factor = 1

Kf for lauric acid = 3.9 oC/m

Using,

dTf = iKfm

      = 1 x 3.9 x 0.0703

      = 0.27 oC

melting point of mixture = 43.2 - 0.27 = 42.93 oC