Objective
To investigate the relationship between the molality
and the boiling point of a solution.
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.
Neverthless, 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 descreased 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).
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
-
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
