Lab Sesion 3 - Boiling point elevations for solutions of benzoic acid in acetone

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

Hypothesis:

When more solute (benozoic acid) we add to the acetone, the vapor pressure of the solvent is less than the vapor pressure of the pure solvent. The boiling point of a solution will be higher than the boiling point of the pure solvent because the solution (which has a lower vapor pressure) will need to be heated to a higher temperature in order for the vapor pressure to become equal to the external pressure. In this case, when more benzoic acid we have in the solution, more it will take to boil.

Table:

Mass of benzoic acid in solution (g)	Molality (mol/kg)	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	56.0	56.4	56.2	0.2
0.5	0.82	58.8	58.0	58.4	2.4
1.0	1.64	60.7	60.5	60.6	4.6
1.5	2.46	51.5	51.3	51.4	5.4
2.0	3.28	65.3	64.7	65.0	9
2.5	4.0	67.1	67.1	67.1	11.1

Evaluation:

We think it was a complex experiment because we confused many times with the tested and this makes that we couldn´t finished correctly the experiment.

The first problem was to calculate the measurement because we calculated using the eye line and this is not very accurate. A good solution is to change the mL into grams so we can calculate it in the balance and we have the exact measure, but this will takes us lot of time and it would slow down our experiment and we don´t have so much time.

When we were going to make the graph using the table, we saw that there were two wrong results in the table, the one using 1.5 mass of benzoic acid in solution. To don´t put this mistake in the graph we remove it and we don´t use it in the graph.

Other big problem was when the liquid evaporate we lose solution and this affect directly to the project and when the acetone boils, and starts to grow bubbles, we also lose liquid and the concentration. We have to take care and being precise when we calculate the temperature because it could react and we can lose liquid because of the bubbles or if it boils, the acetone will evaporate and we also lose liquid, we will have other results.

Also a problem was when we poured acetone in the bunk and this affected in the temperature so we could have different results. To solve this problem, we have to be precise with the temperature because we know that the bunk is not going to be at the temperature we have, so a good idea would be, turn to the higher temperature and pay attention to the thermometer and the temperature it marks when it reacts. Finally we have to be precise with the temperature because when we get the test tube outside from the bunk the temperature will cool down, so we have to see when it reacts and see the correct temperature when the test tube is in the bunk.

Conclusion:

We said in the hypothesis that the more solute (benozoic acid) we add to the acetone, the vapor pressure of the solvent is less than the vapor pressure of the pure solvent, so the boiling point of a solution will be higher than the boiling point of the pure solvent.

In the graph we can see that with more concentration, the boiling point will increase, the line grows up. The function is directly proportional because when x grows also y grows.

When we finished the experiment and we get all the data we could prove that the hypothesis was correct because the boiling point of a solution will be higher than the boiling point of the pure solvent.

References:

Chemspider.com, (2015). Benzoic acid | C7H6O2 | ChemSpider. [online] Available at: http://www.chemspider.com/Chemical-Structure.238.html [Accessed 21 Feb. 2015].

Webbook.nist.gov, (2015). Acetone. [online] Available at: http://webbook.nist.gov/cgi/cbook.cgi?ID=67-64-1&Type=IR-SPEC&Index=QUANT-IR,0 [Accessed 21 Feb. 2015].