Question

CHM 112 Determination of Molar Mass by Freezing Point Depression

Report Sheet and Data Analysis

DATA TABLE

Mass of lauric acid (g)	8.003
Mass of benzoic acid (g)	1.005
Freezing temperature of pure lauric acid (°C)	42.4
Freezing point of the benzoic acid-lauric acid mixture (°C)	38.7

Discuss possible sources of error in this experiment and how those errors would affect the final calculated molar mass. This is an open ended question and a thorough answer is expected. You should identify at least three possible sources of error and discuss the potential effect on the freezing point measurement and the final calculated molar mass.

Answer 1

Formula: T_f = K_f * (w/M)*(1000/W)

Where 'K_f' is the freezing point depression constant of lauric acid

'w' is the weight of benzoic acid

'M' is molar mass of benzoic acid

'W' is the weight of lauric acid

i.e. (42.4 - 38.7) ^oC = 3.9 ^oC.kg/mol * (1.005 g/M) * (1000/8.003 kg)

i.e. M = 132.366 g/mol

The possible sources of error include:

1. The contamination of benzoic acid by moisture to some extent.

2. The measuring balance is not properly tared so that the benzoic acid is weighed more.

3. The student has mistakenly calculated the weight of benzoic acid as little more.