Determining Molecule Weight by Freezing Point Depression Method

Objectives:

1. To determine the freezing point of a substance from its cooling curve.
2. To study the effect of foreign substance content on the freezing point of a solvent.
3. To determine the molecular weight by using the freezing point depression method.

Introduction:

When non-volatile solute is added into a pure solvent, the freezing point of the solvent is depressed. This phenomenon is called the freezing point depression. In other word, the solution possesses a lower freezing point than the pure solvent. The freezing point depression can be explained as solvent molecules leaves the liquid phase and join to form the solid phase; they leave behind a lesser volume of solution where the solute particles may present. This results in the decrease of entropy of the solute particles. Thus, we can say that freezing point depression depends on the concentration of the solute particles present and it is called colligative properties. The colligative properties are solution that depends on the number of molecules in a constant volume of solvent but not the properties of the molecules. However, when the concentration of the solute becomes larger, where the interaction of solute becomes more important, therefore, the freezing point depression may depend on certain properties of the solute rather than the concentration.

In this experiment, we will investigate the phenomenon of freezing point depression and determine the molar mass of substance X. The relationship between the lowering of the freezing point and the concentration of the solution is given by the following:

Methods:

A) Determination of the Freezing Point of Naphthalene

1. A weighing boat was weighed.
2. 5g of naphthalene was added to the weighing boat and weighted again.
3. The naphthalene was transferred into a test tube.
4. The naphthalene was melted using the water bath.
5. When all had melted, the test tube from the water bath was dried and clamped on a retort stand and the initial temperature was recorded.
6. The liquid naphthalene was stirred continuously and the temperature was recorded every 15 seconds until no more change in the gradient for the temperature vs. time graph.
7. A graph of temperature versus time was plotted.
8. The freezing point of naphthalene was determined.

B) Determination of the Freezing Point and Molecular Weight of a Substance

1. Approximately 0.5g of substance X was weighed.

2. The substance was added to the naphthalene in a test tube which was used in part A.

3. The melting and cooling steps were carried out.

4. The molecular weight of substance X was calculated.

[K_f for naphthalene is 6.8]

Apparatus and Materials:

• Naphthalene
• Substance X
• Test tubes
• Stopwatch
• Beaker
• Thermometer
• Glass rod
• Retort stand
• Water bath

Results:

Part 1

Mass of weighing boat 1 = 2.5402g

Mass of naphthalene + weighing boat 1 = 7.5438g

Mass of naphthalene = 7.5438g – 2.5402g

= 5.0036g

Part 2

Mass of weighing boat 2 = 0.4515g

Mass of substance X + weighing boat 2 = 0.9502g

Mass of substance X = 0.4987g

Table 1: The temperature readings for naphthalene.

Table 2: The temperature readings for naphthalene and substance X.

Calculations:

K_f for naphthalene = 6.8 kg mol^-1

Mass of substance X = 0.4987g

Mass of solvent in kg (naphthalene) = 0.0050036kg

Molecular weight of substance X = 677.55g mol^-1

Discussion:

The application of freezing point depression is the determination of the molecular weight of the substance X. A weighed amount of the solute (substance X) is dissolved in a known mass of solvent (naphthalene). The freezing point of the solvent (the temperature at which solid and liquid phases are in equilibrium) is determined by the cooling of the solution. When the graph of the time versus temperature was plotted, a longest horizontal portion which is the constant temperature of the graph indicates the freezing of the pure liquid. However, a solution (mixture of substance X and naphthalene) will freeze over a range of temperature which is lower than the constant freezing point of the solvent (naphthalene). The plot will show a change of slope when solid solvent begins to form. The concentration of dissolved solute will steadily increases as the solvent freezes. This will cause the freezing point to continue decrease after the constant temperature. After obtaining the change of freezing point (∆T) and the value of K_f, then it is possible to calculate the molar mass of substance X.

Based on the graph plotted for the pure solvent (naphthalene), the freezing point obtained is 76.0^oC. Whereas, the freezing point obtained for solution of substance X and naphthalene is 75.0^oC. From the results obtained in the experiment, it obeys the theory of freezing point depression stating that the solution which contains naphthalene and substance X will have lower freezing point compared to the freezing point which contains only pure solvent (naphthalene). All the graphs used to determine the freezing point shows a staircase-like shape. For the graph of temperature versus time for naphthalene, initially, the temperature was 76.0^oC and the temperature remained constant for more than 3 minutes. After that, the temperature continues to fall until the last second the time was recorded. The shape of this graph did not obtained like a staircase as the initial temperature start to become constant and continue to fall until the end of the experiment. During the starting of the experiment, the temperature of the solvent does not decrease because the solvent was not over heated in the water bath. Thus, the temperature measured in 0 second is the freezing point of the solvent. However, the graph for the solution of naphthalene and substance X exhibits a staircase-like shape. The initial temperature was 80^oC then there is a slight decrease until it reaches 75^oC, the equilibrium state of the substance which is the conversion of liquid to solid state. After that, the temperature will start to decrease until the end of the experiment.

The experimental freezing point was selected based on the temperature that remains the longest period for all the three different solution. This is because during this period, temperature do not rise until all the solid has melt as heat of fusion is taken up to convert the solid state substance to liquid state.

In order to make sure the results obtained obeys the theory, some of the precautions steps should be taken in order to prevent results error. Firstly, all apparatus used should be washed and rinsed thoroughly with distilled water to avoid contamination occurrence. Furthermore, during the solution is left to melt from solid to liquid, the content should be stirred in order to maintain thermal equilibrium. Uneven distribution of heat in the solution will caused the temperature obtained not accurate. Thus, the freezing point depression will be affected. In order to dispose the substances inside the test tube, the substances was melted in the water bath and disposed into the fume chamber.