Researcher

Rida Atif

Objective

To determine the recrystallization yield of benzoic acid in 12mL of ethanol and 20mL of water. This ratio is in accordance to Lab02

Procedure

1.0153g of benzoic acid and 12mL of ethanol were added into a 50mL erlenmeyer flask. The flask was set to boil until the acid dissolved. The flask was removed from the heat, and 20mL of water were added into the solution. The flask was set to boil again until the acid dissolved again, and was removed afterwards and set to cool down over night so crystals could form. The crystals were filtered through a suction filtered and massed.

Results

Calculation Sheet
EXP304a-HNMR
EXP304b-HNMR (this NMR was taken after the sample was run through the vacuum pump)
Initially1.1257g of benzoic acid (EXP304a) was yielded.
After drying the sample further using a vacuum pump, 0.6227g of the solute was yielded (61.3% of initial mass)

Discussion

The crystals ended up weighing more after being filtered than the initial amount, so it was suspected that the crystals were not completely dried when taken the mass of.
The NMR spectrum for EXP304a shows that there is still solvent in the sample. In order to address this problem, the sample will be dried using a vacuum pump
The NMR spectrum for EXP304b shows that there is no water or ethanol left in the sample; the benzoic acid crystals are completely dry
The [[#|paper]] that this experiment was based off of had a recrystallization yield of 41%
The temperature curve (for ethanol/water 40:60vol) predicted that at boiling point, the solubility of the solution would be 2.21M with a room temperature solubility of 0.19M (this means the predicted recrystallization yield would be 91.4%)
The solubility at boiling point for this experiment was 0.25M, with a predicted recrystallization yield of 25%.
The actual yield after drying it through a suction filter was 110% which is unrealistic. After drying the sample again through a vacuum pump, the yield was 61.3%.
A portion of the low end yield could be accounted for the loss of sample as it was transferred from container to container because it each time not exactly all of it was able to be taken out.
Due to the large discrepancy between the predicted recrystallization yield and the actual yield, it is possible the room temperature solubility is too high and should be around 0.1M

Conclusion

The solvent selector did not do a [[#|good job]] in predicting the solubility of the solution at boiling point.
This experiment should be repeated with the ethanol/water being mixed already instead of doing one solvent at a time and only drying the sample once in a vacuum pump to compare results.
The solubility at room temperature should be measured experimentally to confirm whether the 0.19M value is too high or not.

Log

2012-06-27

12:14 - Turned hot plate dial to "7"
12:24 - Added 1.0153g of benzoic acid into 50mL Erlenmeyer flask.
12:36 - 12mL of ethanol was added into the same flask, a loose cap was put on, and was set on hot plate.
12:39 - All of the crystals have dissolved into the ethanol and the flask was removed from the hot plate.
12:54 - No crystals appear to form in flask
12:55 - 20mL of water was added into flask. white crystals formed as soon as water entered flask
12:55 - Flask was set on hot plate again to boil and dissolve crystals.
12:57 - All of the crystals have dissolved into the ethanol/water mixture
12:58 - The flask was removed from the hot plate and set to cool.

2012-06-28

12:29 - Filtered the contents of the flask through a suction filter. The crystals look like flakes.
12:43 - Mass of empty vial with cap: 13.5473g
12:48 - Mass of vial and cap with recrystallized benzoic acid: 14.6730g
12:49 - Mass of benzoic acid recrystallized (EXP304A): 1.1257. This does not make sense since the initial amount was 1.0153g.
12:56 - A solution was prepared with CDCl3 and 0.102g of the benzoic acid, so that EXP304A-NMR1 could be taken of the sample. Inside the NMR tube, there were droplets of water on the walls.

2012-07-11

8:45 - Transferred EXP304A into a 25mL round bottom flask and set in vacuum pump

2012-07-12

9:05 - The round bottom flask was removed from the vacuum pump.
9:20 - Mass of vial with cap: 13.7139g
9:30 - The dry crystals (EXP304b) were transferred to a vial
9:31 - Mass of vial, cap, and exp304b: 14.3366g
9:40 - Prepared EXP304b-HNMR with CDCl3, and took NMR spectrum of it.