LAB REPORT SESSION 3: The elevation in the boiling point of acetone when applying benzoic acid

Objective:

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

Hypothesis:

We think that the more solute (benzoic acid) we add to the pure solvent (acetone), the higher its boiling point will be because when a solute is added to a liquid, the solutes blocks the solvent particles from reaching the surface, so the solvent particles will apply a lower vapour pressure than with the pure solvent. This means that, to overcome the external pressure made by the air, the solvent particles would need more heat as we add more solute to have the enough energy to escape the container they are in and start boiling. (Canning, O. 2015) Also, we can observe this in the equation of change in boiling point :

DTb = Kb ·m

The more solute you include, the higher molality the solution would have and a higher increasing change in boiling point that it would be.

Table: Relationship between the change in the boiling point and the molality of the solution.

Mass of benzoic acid in solution (g)	Molality (m)	First run - Boiling point (oC)	Second run - Boiling point (oC)	Average boiling point (ºC)	Change in boiling point compared to pure acetone (oC)
0	0	56	56,4	56,2	0,2
0,5	0,82	58,8	58	58,4	2,4
1	1,64	60,7	60,5	60,6	4,6
1,5	2,46	51,5	51,3	51,4	- 4,6
2	3,28	65,3	64,7	65	9
2,5	4,1	67,1	67,1	67,1	11,1

Graph:

Conclusion:

As we can observe clearly in our results, without taking on account the fourth result, which clearly an anomaly as it doesn’t follow the pattern like the rest of the results of an increasing straight line, the more mass applied of benzoic acid inside the solution, the higher its boiling point will be. As we explained before in the hypothesis, this is caused due to the block of the acetone particles by the acid to be able to float to the surface, so it needs to be an increase in the temperature to balance with the low pressure applied by the acetone and the higher boiling point that the benzoic acid particles have. (Canning, O. 2015). We can observe this in the equation of change in boiling point :

DTb = Kb ·m

The more solute you include, the higher molality the solution would have and a higher increasing change in boiling point that it would be.

Evaluation:

Throughout the experiment there have been some obvious problems which have caused the results, such as the fourth one when applying 1,5 g of benzoic acid, to alter completely and give results which didn’t followed what should really happen in the experiment. Some of this problems could be:

-          The difficulties of knowing when the solution starts boiling or not, as we didn’t have any parameters in which we could base on, so in each experiment the boiling point would start in a different state of the solution, and this could have increased or decreased the amount of temperature needed to apply to start seeing the first bubbles (which at first look they are impossible to distinguish). What we could do to solve this problem is record the experiment with the camera focused on the test tube and at the thermometer. After the solution start boiling completely we can go backwards in the video and take a closer look at the surface. Like this we could notice, at a slower motion, when exactly the first bubble appears and, by looking at the thermometer, at what exact temperature.

-          The benzoic acid didn’t dissolve completely in the acetone, either because the amount of steering wasn’t enough or because some of the solute laid on the bottom of the test tube, which can cause the temperature needed to boil the solution be lower as there is less solute blocking the surface of the acetone, and most of the solute is beneath, leaving the path for the acetone particles to apply a higher pressure. A solution for this could be to determine a specific time needed to steer the solution, also steering by touching the bottom of the test tube always, and like this all the experiments will have the same amount of solute dissolved and the acetone will apply the same pressure, so there will be no difference in the results of each experiment.

Bibliography:

Canning, O. (2015). Colligative properties. [online] Sciencesfp.com. Available at: http://www.sciencesfp.com/unit-3---colligative-properties.html [Accessed 21 Feb. 2015].   

University, P. (2015). Boiling Point Elevation. Chem.purdue.edu. Retrieved 20 March 2015, from http://www.chem.purdue.edu/gchelp/solutions/eboil.html