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Solubility book (3rd Edn)
To determine the temperature at which different concentrations of 4-fluorobenzoic acid in ethanol solution will come out.The difference between this and the last experiment is that I used lower concentrations in order to dissolve at lower temperatures, hopefully yielding better results.
Five samples of solution were prepared with ethanol and acid. Five half dram vials were filled with 0.4g ethanol each, then a known amount of acid was added to each vial to produce the desired concentration. First, each empty glass vial was weighed and the scale zeroed. Ethanol was then added to the vial until it weighed correctly. The exact weights are recorded in the results. After all five vials contained the solvent, the solute was added. We used wax paper to weigh specific amounts of 4-fluorobenzoic acid and then scoop/pour into the appropriate marked vial. Now we have 5 marked vials containing different concentrations of acid and solvent. We covered the vials in parafilm and vortexed them for 30 seconds. We tagged the vials with manilla and placed them in a heated bath. Samples A1-A5 were dropped into a water bath heated to temperature 48 degrees and then lowered in increments to determine the temperature at which precipitate forms.
[supernatant of saturate solution? JCB] yes
Intrapolated value 1- has been entered into SolSum sheet
[After you add an entry to the SolSum sheet hit the dropdowns for solute and solvent to make sure your new value gets included - in this case you made a spelling error and it did not get merged JCB]
[It looks like you obtained good results - add them to the solsummix sheet and intrapolate to 23C to get the room temp measurement and put that in the solsum sheet - you will find these on the bot page from the navigation bar lower left JCB]
[You just need to take an NMR of the supernatant of one of the saturated solutions and your lowest concentration sample for confirmation by SAMS JCB]
Actual concentrations may be higher than calculated due to the quick evaporation of solvent, which can be the reason for difference in SAMS and original calculations.
[What numbers are you comparing? JCB]
I am comparing sample A1 concentration (0.24) calculated on the spreadsheet for the experiment with the SAMS concentration (0.26) calculated from the SAMS spreadsheet.
[The SAMS value is actually 0.29M - the initial peak limits were too narrow JCB]
[How does your SAMS measurement of A5 compare to the intrapolated value? JCB]
My SAMS measurement is 0.85 and the intrapolated value, at 23 degrees room temperature, would be 0.89. These values are close enough to lead me to believe they are correct.
My results indicate that solubility of this compound at 23 degrees room temperature is between 0.85M and 0.89M. Exp121Plot can be used to intrapolate solubility at different temperatures. My NMRs, HNMR-A1 and HNMR-A5, sustain this method as an accurate way to obtain solubility data. Further, sample A1 from
provides a data point at -15 degrees. A1 concentration was 0.49M, so that fits within the intrapolation curve of Exp121, providing an additional check.
[use military time JCB]
15:08-weighed ethanol solvent into vials
15:25-weighed solute acid into vials
15:39-parafilmed and vortexed samples: A1 and A2 dissolved; A3 partially dissolved; A4 and A5 saturated
15:50-tagged and placed in 48 degree bath
16:07-sample A3 fully dissolved; A4 and A5 partially dissolved
16:12-thermostat set to 40 degrees
16:28-set to 37 degrees [also record the actual temperature of the bath at all these times JCB] actual temperature usually same as previous thermostat setting
[It takes time for temperature to change - it is good practice to also note the actual bath temperatures JCB]
16:35-set to 34 degrees
16:51-set to 30 degrees
17:01-set to 27 degrees; noticed crystal formation in samples A4 and A5 [but you didn't indicate that A4 and A5 ever went into solution - so how did you report that A4 came out at 30C in the spreadsheet? JCB] A4 went part way in, so I estimated based on my visual perception when the sample came out. I say that it came out because I noticed additional crystals forming and significantly more solid precipitate than when we started
[No you can't use the temperatures unless all solute has dissolved at a higher temperature JCB]
17:08-set to 22 degrees
17:32-set to 18 degrees
17:43-set to 15 degrees
17:50-set to 12 degrees
17:55-set to 8 degrees
18:03-set to 4 degrees; crystal formation at 8.5 degrees in sample A3
18:16-set to -2 degrees
18:27-set to -5 degrees
18:33-set to -9 degrees
18:45-set to -15 degrees
18:52-set to -18 degrees; crystal precipitate formation at -12 degrees in A2
19:05-set to -24 degrees
19:18-set to room temperature; no precipitate in sample A1
14:45-NMR's taken of A1 and A5
[How did you separate the supernatant and what temperature? JCB]
Supernatant was separated at room temperature by centrifuging the sample then extracting the supernatant with pipette.
[In the discussion you say that solvent might have evaporated - how exactly did you transfer the supernatant? JCB]
Used pipette to extract supernatant from centrifuged sample. Some solvent might have evapoated when the lid was off during pipetting. When weighing the solvent, in a container with no lid, I noticed that the weight was slowly decreasing and I attributed this to quick evaporation.
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