Boiling point
elevations for solutions of benzoic acid in acetone
Objective: The aim of this
experiment is to investigate the relationship between the molality and the
boiling point of a certain substance.
Hypothesis: As we increase the
solute (which makes molality increase), the boiling point of the solution
increases. This means that the boiling point elevation compared to pure acetone
increases too.
I make this assumption because, when molality increases the mol/kg
increases. It is harder to evaporate a solution which has more concentration of
solute. It doesn't matter the type of solute used, but the quantity. If we
increase the solute (benzoic acid) , the solvent molecules (acetone) will have
more difficulties to evaporate. The solute molecules will act as a “barrier”
without letting the solvent evaporate until the it is broken up.
Molality values used
in the experiment
0 g benzoic acid in 5 g acetone – 0 mol/kg
0.5 g benzoic acid in 5 g acetone - 0, 82 mol/kg
1.0 g benzoic acid in 5 g acetone - 1, 64 mol/kg
1.5 g benzoic acid in 5 g acetone - 2 ,46 mol/kg
2.0 g benzoic acid in 5 g acetone – 3,28 mol/kg
2.5 g benzoic acid in 5 g acetone - 4,09 mol/kg
Experiment Results
Table Results: Relationship between molality,
mass of benzoic acid and boiling point elevation compared to pure acetone
|
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
|
-4,7
|
|
2.0
|
3,28
|
65.3
|
64.7
|
65.0
|
9
|
|
2.5
|
4,09
|
67.1
|
67.1
|
67.1
|
11.1
|
Conclusion
So, once we have finished the experiment and we have collected all the
results in a table and a graph, we can clearly observe how our hypothesis was proved to be true. As we increased the molality of the solutions, the boiling point
increased at a similar rate. This means it is a directly proportional relationship.
As molality increases, the average boiling point increases, but the
boiling point elevation compared to pure acetone too.
However, there is an anomaly in the results. When we add 1,5 g of
benzoic acid in the acetone, the boiling point decreases 4,7 degrees from the
boiling point of acetone. We haven´t used it in the graphs as it
would have ruined our conclusions. It isn’t an important error as it was just 1
over 6 different solutions.
Evaluation
The anomaly mentioned in the conclusion was produced, probably, by a failure in the procedure or even a human error. This could have been prevented if we had checked the results with each other, or or with the teacher and our peers. We could have redone the anomaly, but we decided to stick with our results, as we were running out of time and we did not want to risk losing a result.
The scale we used to mass the benzoic acid was constantly being used by everyone, so it had remains of other materials (some groups were massing sugar), which could have altered the accuracy of the measurements, leading our results to be wrong. Although we were careful and pressed the reset button to avoid that.
References
Chuck, W. (2014). How does molality affect the boiling point? |
Socratic. Socratic.org. Retrieved 22 February 2015, from
http://socratic.org/questions/how-does-molality-affect-the-boiling-point
Cush, I. (2009). Fun With Colligative Properties:. Misterguch.brinkster.net.
Retrieved 22 February 2015, from
http://misterguch.brinkster.net/colligativepropertiestutorial.html
