Lab Session III - Freezing Point Depression

Objective

To investigate the relationship between the molality and the boiling point of a solution.

Hypothesis

In this type of experiments, when we use a solution instead of a pure liquid it is more difficult to form a nice regular structure. For this reason, we must cool the solution to a lower temperature in order for it to freeze. 

In other words, the more solute is added to the solution, the lower the freezing point will be. In this experiment, we are gradually adding grams of sugar to our water-sugar solution. According to the theoretical background, the bigger is the mass of sugar we add to the solution, the lower the freezing point should be.

Results Table 

Mass of sugar in solution (g)	Molality (mol/kg)	Attempt 1 - Freezing point (oC)	Attempt 2 - Freezing point (oC)	Average freezing point (oC)	Change in freezing point compared to pure water (oC)
0	0	0	-0.6	-0.3	-0.3
0.5	0,2	-0.6	-0.8	-0.7	-0.7
1.0	0,4	-0.3	-0.1	-0.2	-0.2
1.5	0,8	-1.7	-1.7	-1.7	-1.7
2.0	1	-2.0	-2.2	-2.1	-2.1
2.5	1,4	-2.9	-2.5	-2.7	-2.7

Graph

Conclusion

Observing the previous graph we can see that, in most cases, as the molality (mol/kg) increases, the freezing point decreases (ºC). There is only one case in which the freezing point increases as the molality increases: when m= 0,4 and f.p= -0,2. This could be due to an error committed while carrying out the experiment.

Nevertheless, these results match completely with the hypothesis and the theoretical background. According to these, the more solute is added to a solution the more difficult it will be for it to freeze. In other words, the bigger is the molality the more difficult it is for the solution to create a regular structure and therefore it needs to be cooled at a lower temperature.

This is exactly what has happened in this experiment. As we have increased the molality from 0 to 1,4 mol/kg, the freezing point has also gradually decreased from -0,3 to -2,7 ºC. This is clearly observable in the graph above, where the line goes down along the Y axis (freezing point) as we go along the X axis (molality).

Evaluation

The first possible error is that, as we were using an ice bath to cool down the solutions, we weren’t able to control the temperature very well. The temperature varied as the time went onbecuase the ice melted and, consequently, the water started to warm up. A solution to this problem could be to have a separate ice bath for each experiment. This way, we wouldn’t need the ice to be frozen for so long. This would provide us with more controlled temperatures.

Another error that may have occurred is that we didn’t really know when the solution had actually frozen because the thermometer was inside the test tube and as it could move, we thought that it was still liquid. This might have given us unreliable results. A solution to this would be to check if the solution has frozen with a thin and long wooden stick that we could introduce in the test tube.

References

-          Hyperphysics.phy-astr.gsu.edu,. (2015). Freezing Point Depression in Solutions. Retrieved 21 February 2015, from http://hyperphysics.phy astr.gsu.edu/hbase/chemical/meltpt.html

-          Hyperphysics.phy-astr.gsu.edu,. (2015). Freezing Point Depression in Solutions. Retrieved 21 February 2015, from

http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/meltpt.html

-          Chemistryexplained.com,. (2015). Colligative Properties - Chemistry Encyclopedia - water, uses, examples, gas, number, equation, salt, property. Retrieved 21 February 2015, from http://www.chemistryexplained.com/Ce-Co/Colligative-Properties.html