Researcher

Rida Atif

Objective

To determine the recrystallization yield of benzoic acid in ethanol/water(40:60)vol. based on the solubility predicted by the temperature curve.

Procedure

An empty erlenmeyer flask and cap were massed. About 5g of benzoic acid were added into the flask and it was massed again. 12mL of water and 8mL of ethanol were measured out based on their weight and density and added into the flask one at a time. The whole flask was then set on the hot plate until the solute dissolved completely in the ethanol/water mixture. Once it was completely dissolved, the flask was removed from the hot plate and set to cool down over the weekend. The crystals were filtered through suction filter, but that did not remove all of the solvent, so the semi-wet crystals were transferred to a few vials and dried in a dessicator attached to a [[#|vacuum pump]]. The dry crystals were collected and massed.

Result

Calculation Sheet
exp318a-HNMR
The solubility at boiling point for this solution was 1.803M
With this boiling point solubility and a room temperature solubility of 0.19M, the predicted recrystallization yield was 89.4%
The actual recrystallization yield was 83.7%

Discussion

In Lab02, the ethanol and water were mixed in a 12:20 volume and were able to dissolve 1g of benzoic acid. The solubility at boiling point for that experiment was 0.251M, and the experimenters were able to recrystallize 41% of the benzoic acid.
According to the temperature curve, the solubility at boiling point is 2.21M, and although the ratios of solute and solvents are based off this solubility, the solubility in this experiment turned out to be 1.8M at boiling point.
The temperature curve predicted a recrystallization yield of 91.4% with a solubility of 2.21M and the 1.8M solubility predicted a solubility of 89%. The actual yield was about 84% which is very close to the predicted.
Model003 predicted that an ethanol/water mixture in 40:60 volume would be a fairly good solvent to recrystallize benzoic acid from, given that the boiling point solubility was 2.67M and the room temperature solubility was 0.33M. In this case, the predicted recrystallization yield would be 87.4%.
EXP318a-HNMR shows that there is no ethanol or water left in the sample of benzoic acid. The crystals are completely dry and the yield is accurate. Furthermore, the yield is also accurate because less than 0.2% of the solvent evaporated overnight which should not affect the yield.
Additionally, it is a huge limitation if only a suction filter is available to dry the sample because it is inconvenient. Water is very difficult to dry completely from crystals and a high [[#|vacuum pump]] is better suited for this [[#|job]].
Lastly, this experiment done in this particular molarity (5g of benzoic acid to 12mL of water and 8mL ethanol) because there was a large discrepancy between the concentration of the solution in exp317 which was based off Lab02 and the temperature curve.

Conclusion

Although there is a small difference between the solubility at boiling point that the temperature curve predicted and the experiment actually showed, ethanol/water(40:60)vol is a good solvent mixture to recrystallize benzoic acid from; however, this solvent mixture is inconvenient to use because it is difficult to completely dry the water from the crystals.
Furthermore, although model003 predicted a good recrystallization yield but with different solubility values, it still predicted accurately that this solvent mixture would be a good one to recrystallize benzoic acid from.

Log

2012-07-13

9:29 - Turned hot plate on to 170C
10:06 - Mass of empty 50mL Erlenmeyer flask and cap: 51.8505g
10:10 - Mass of flask, cap, and benzoic acid: 57.2423g
10:19 - Measured 12g of water and added into flask (12g of water is equivalent to 12mL of water with a density of 1.00g/mL)
10:28 - Measured 6.25g of ethanol and added into flask (6.25g of ethanol is equivalent to 8mL with a density of 0.78g/mL).
10:34 - Set the flask on the hot plate
10:38 - Turned hot plate up to 200C
10:50 - All of the benzoic acid has dissolved into the ethanol/water mixture. The flask was removed from the hot plate and set to cool down.
10:55 - Mass of flask, cap, crystals, and solvent:75.440g
10:56 - Covered flask with parafilm to prevent solvent from evaporating while cooling down
10:57 - Mass of flask, cap, crystals, solvent, and parafilm: 76.288g

2012-07-16

9:50 - Filtered the contents of the flask through a suction filter
10:31 - The crystals are still a little wet.
10:34 - A small amount of the crystals were put into a small vial with chloroform
10:41 - The liquid from the vial was transferred to a NMR tube, but drops of water stayed on the walls of the tube. There is no reason to take an NMR at this point.
10:50 - Mass of empty vial 1: 11.7090g
10:50 - Mass of empty vial 2: 11.7345g
10:50 - Mass of empty vial 3: 11.7708g
10:53 - the vacuum of the suction filter was turned off and the crystals were put into a few different vials due to the large amount of solid.
11:00 - The openings of the vials were covered in parafilm and several, small holes were poked into the parafilm
11:29 - The three vials were put inside of a dessicator, and the dessicator was attached to a high [[#|vacuum pump]].
2:33 - The vacuum pump was stopped and the 3 vials were removed from the dessicator
2:34 - Mass of vial 1 with benzoic acid: 13.4056g
2:34 - Mass of vial 2 with benzoic acid: 13.3242g
2:34 - Mass of vial 3 with benzoic acid: 12.9975g