Determination of Molar Mass by Freezing Point Depression

Chem 112 

Formal Report#2 

Title: 

Determination of Molar Mass by Freezing Point Depression

Introduction: 

In this experiment, the molar mass of an unknown liquid was determined by measuring the freezing point depression of the unknown, dissolved in pure cyclohexane. Freezing point depression is defined as the decrease of the freezing point of a solvent (cyclohexane) on the addition of a non-volatile solute (the unknown). The extent of lowering of the freezing point is dependent on the nature of the solvent and the molal concentration of the solution. The freezing point of the solution differs from that of the solvent because it exhibits different properties. The freezing points of the solvent, and solutions were determined from a cooling curve (a graph that outlines the cooling of a liquid, incorporating phase changes). The freezing points were used to determine the freezing point depression values, which were then used in the calculation of the molality of the solutions. The relationship was defined by the equation: 

Δ Tf  = Kf m  

The freezing point depression constant (20.8) is represented by Kf, the freezing point depression by Δ Tf, and the molality of the solution by m. Being as the molality of a solution is equal to the number of moles of solute divided by the mass in kilograms of solvent (previously weighed), the number of moles of solute was determined and therefore; the molar mass calculated from the relationship of number of moles being equal to mass of substance (previously weighed) over its molar mass. 

Procedure: 

A warm temperature bath and ice water bath were first set up. Cyclohexane was then poured into a test tube which was fitted to the stopper with temperature probe and stirrer. The apparatus was placed into the warm bath then the ice bath then the temperature was recorded.

The unknown liquid was then added to the cyclohexane and the mixture was quickly weighed using a top-loading electronic balance to determine the weight of only the unknown liquid. The same procedure of placing the apparatus in the warm bath, then cold bath and recording the temperature was repeated with the unknown liquid mixture. The procedure was, again, repeated once more using new samples of unknown liquid and cyclohexane. Cooling curves for pure cyclohexane and the two solutions were plotted and the freezing point depression was used to determine the molar mass of the unknown liquid. 

Datasheets:

Table showing the masses of substances used

Results and Calculations:

Table showing freezing point depressions for each trial and the calculated molecular weight of the unknown

• Freezing Point Depression = Final Temperature - Initial Temperature

Trial 1: 3.70- 6.40 = -2.70  C

Trial 2: 4.25 - 6.40 = -2.15 C

• Molality of Solution: Change in Temperature / Constant

Trial 1: (2.70)/20.8 = 0.130 mol/kg

Trial 2:  (2.15)/20.8 = 0.103 mol/kg

• # Moles of Unknown: molality x kilograms of solvent

Trial 1: 0.130 x 0.0187 = 2.43  x 10^-3 mol

Trial 2: 0.103 x 0.0188 = 1.94 x 10-3 _mol

• Molecular Weight (Molar Mass) of Solute: Mass of solute/ # Moles

Trial 1: (0.54)/2.43  x 10^-3 = 220 g/mol

Trial 2: (0.46)/1.94 x 10-3 _{= 240 g/mol}

• Average MW: Sum MW/2

Average = (220 + 240)/2 = 230 g/mol

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