How molality affects freezing point.

Objective: To find how molality changes the freezing point of water, to do this, we will add sugar to water.

Hypothesis:  Using the FP formula, the FP should go down, this happens because molality is increased, as more sugar we add, higher molality we get and therefore we get a lower freezing point, also, the first one should be at 0ºC as that is the water's freezing point. The formula for FP is Change in FP=kc*molality, the kc stays the same while molality goes up, if the molality without sugar is 0, the freezing point would be 0, however, if when adding sugar, we get a molality of 1, we would get -1,86 degrees as water's new FP.

1.       What are the molalities of each sugar solution: C6H12O6.

72+12+96= 180

0 g sugar in 5.0 g water –  Pure water so at 0

0.5 g sugar in 5.0 g water –  0,56 mol/kg

1.0 g sugar in 5.0 g water –  1,11 mol/kg

1.5 g sugar in 5.0 g water –  1,67 mol/kg

2.0 g sugar in 5.0 g water –  2,22mol/kg

2.5 g sugar in 5.0 g water –  2,8 mol/kg

2.       Place the test tubes in a salt ice mixture and note at which temperature each solution freezes. How will you know it is freezing? 

Because solid will start to appear, also, the thermometer  will start go get stuck.

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(anomalous)	0.3- 0	0
0.5	0,56	0.6	0.8	0.7	-0.7
1.0	1,11	0.3	0.1	0.2	Anomalous
1.5	1,67	1.7	1.7	1.7	-1.7
2.0	2,22	2.0	2.2	2.1	-2.1
2.5	2,8	2.9	2.5	2.7	-2.7

3. Plot a graph of molality (x-axis) against the freezing point (y-axis).

I deleted the 0,2 as it was anomalous, I instead used 1,2 (which was what there is between 0,7 and 1,7, as well as making sense, 0-0,56à0,7. 1,67-2,22à 0,4.

Conclusion:

As the hypothesis stated, as we added more sugar we decreased the freezing point, this is also proven by the freezing point formula. We could see, as we added more and more sugar, even if we added the same amount of sugar (1g to 1,5g to 2g) the decrease in freezing point was lower. We also got anomalous results, with 1 gram, we got both anomalous results.

Evaluation:

For the 1g, we should have done it at least one more time, if it kept being anomalous, we should have to try and find the problem, if it followed the path, we would have probably tried a 4th time to make sure it has to follow the path (which is supposed to). It is possible this isn't anomalous, and it's the correct one, and the rest are anomalous, probably caused by problems in human calculations (lack of precision) as putting too much sugar, or waiting too much time. If this was anomalous (which is how it looks like), it was probably putting too much water or not enough sugar, it's strange to end the experiment too early, as you can feel when it's frozen and when it's not, however, it's easy to leave it too long. We also got 0,3 in pure water, this has been tested a lot of times, and it's definitely anomalous, probably caused by waiting too much time, and having it frozen for already 0,3ºC, however, it should be repeated. In general, what I would have done is to repeat the experiment a 3rd time to make sure of results, the way we are doing it, one anomalous result can mess up the experiment.

References:

About.com Education, (2015). Calculate Freezing Point Depression. [online] Available at: http://chemistry.about.com/od/workedchemistryproblems/a/Freezing-Point-Depression-Example-Problem.htm [Accessed 23 Feb. 2015].

 Images.tutorcircle.com, (2015). [online] Available at: http://images.tutorcircle.com/cms/images/44/periodic-table.PNG [Accessed 23 Feb. 2015].