The molecular mass of lauric acid was determined experimentally by finding the f

Question

The molecular mass of lauric acid was determined experimentally by finding the freezing point depression of stearic acid when lauric acid was dissolved in it. For stearic acid, K_f is 4.5 degree C kg/mol and freezing point was determined to be 68.50 degree C. When 2.000 g of lauric acid was dissolved in 10.000 g stearic acid, the freezing point of the solution was determined to be 64.10 degree C. (Answers below must show complete solution to receive credit.) What is the freezing point depression (delta T_f) of stearic acid due to the presence of lauric acid? What is the molality of the lauric acid in stearic acid solution? How many moles of lauric acid are present in the solution?

Explanation / Answer

a) dTf = T0 - Ts = 68.5 - 64.1 = 4.3 degree C
b) dTf = Kf * m
4.3 = 4.5 * m
molality = 4.3/4.5 = 0.9555
c) molality = number of moles of solute/mass of solvent in kg
0.9555 = n*1000/10
n = 9.55 * 10^-3
d) n = mass/molar mass
9.55*10^-3 = 2/M.W.
Molar mass = 2/(9.55*10^-3) = 209.42 g/mol

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