LAB REPORTS- PHYSICS AND CHEMISTRY

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:

Our hypothesis is that the mass of our solute, benzoic acid and molality will have a directly proportional relationship between them. This hypothesis is due to our thoughts of what we think is going to happen in the experiment. We believe that the mass of benzoic acid is going to increase as well as the molality will increase too (due to our thoughts).

To demonstrate this assumption, we have the original formula of the boiling point:

ATb= Kb  x m                                 Boiling point= ebullioscopic constant x molality

If we multiply a big quantity of molality by Kb, there’s going to be a big change in boiling point, boiling point would grow. And if we multiply a small quantity of molality of Kb, there’s going to be a big in boiling point.

This is probable to happ

en because we are adding more and more solute to the solvent (acetone) each time. This means that the vapour pressure is decreasing (see Raolt’s Law). If we see this law, we’ll discover that liquids boil when the vapour pressure is exactly the same as the atmospheric pressure, so due to this theory, if we add more benzoic, we would need more heat to boil it so boiling point would increase.

Method:

1.     In each of the 6 test tubes, measure 5.0 g acetone.

2.     Leaving one of the test tubes, add 0.4, 0.8, 1.2, 1.6 and 2.0 g of benzioic acid.

3.     Stir the solutions to ensure all the benzoic acid has dissolved.

4.     What are the molalities of each solution:

Results:

Number of moles= grams of solute/ molecular mass

Molality= number of moles of solute/ weight of solvent in kg

0 g benzoic acid in 5 g acetone –  Pure acetone so à 0            49.1

0.4 g benzoic acid in 5 g acetone – 0.71 moles                         56.5º

0.8 g benzoic acid in 5 g acetone – 1.43 moles                         61.4º

1.2 g benzoic acid in 5 g acetone – 2.14 moles                         63.2º

1.6 g benzoic acid in 5 g acetone – 2.86 moles                         64.9º

2.0 g benzoic acid in 5 g acetone – 3.57 moles                         66.3º

Table:

Table to show the relationship between molality and boiling point.

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	49.1	55	52.05	-4,14
0.4	0,7	56.5	60	58.25	2,05
0.8	1,4	61.4	60	60.7	4,5
1.2	2	63.2	63.5	63.35	7,15
1.6	2,8	64.9	65.5	65.2	9
2.0	3,4	66.3	66.5	66.4	10,2

Graph:

Graph to show the relationship between molality and boiling point.

Conclusion:

Due to my results, I can demonstrate that the hypothesis was correct, as at the time that the mass of benzoic acid increased, the molality increased too so that way, my hypothesis was correct.

Due to the formula: ATb= kb * m, as there’s a big quantity of molality there’s a big change in boiling point, so, if molality increases, boiling point should increase too.

I can demonstrate it just observing the graph and the table (the results).

My theoretical results were the same as our results as the directly proportional relationship between mass of solute and molality is demonstrated, the directly proportional relationship between boiling point and molality is demonstrated too. My theoretical results and my results are equal.

Evaluation:

Error: There could have been some moments that we don´t really know if the solute was already boiled or if it was in process of it, in the case that we thought that the substance was not yet on its boiling point, we increase the time of waiting to see if it was already boiled or not, this is because we see some bubbles but not as much as we thought as we start making the experiment.

Solution: Knowing before we start making the experiment how much time we need to wait until the substance is completely boiled.

Error: Possibly, every substance needs a determined quantity of other substance, in order to reach his boiling point. We could have put more or less of the quantity of benzoic acid than needed for it to come into its boiling point of the substance.

Solution: As before, knowing the exact quantity of the benzoic acid that we need to put on the test tube, to calculate the boiling point of the substance.

Error: On the moment that we put a higher temperature to the substance to see how is it boiling point and with this the molality, the process from getting out the test tube from the bath to the place where we need to make the calculation (as for example, how much time does it get to be boiled, how many bubbles could be seen as with this we could know if it is already boiled…),in this process the temperature could decrease as it start transforming to the temperature of the room, and this could have made bad our results because of the change in temperature so quickly.

Solution: Putting the substance at a higher temperature that we need, because when the benzoic acid would start having a decrease on its temperature, the boiling point could occur on the moment and with the temperature that we need to, or even, making all our experiment near the bath, so we could have a higher control on the temperature of the benzoic acid that we need, in order that our results could be good.

Bibliography:

En el texto: (Ausetute.com.au, 2015)

Bibliografía: Ausetute.com.au,. (2015). Chemistry Tutorial : Boiling Point Elevation and Freezing Point Depression. Retrieved 24 March 2015, from http://www.ausetute.com.au/freezing.html

En el texto: (Merriam-webster.com, 2015)

Bibliografía: Merriam-webster.com,. (2015). directly proportional | related so that one becomes larger or smaller when the other becomes larger or smaller. Retrieved 24 March 2015, from http://www.merriam-webster.com/dictionary/directly%20proportional