Jim

I need some help with calculating percent yield and percent purity of aspirin.

Here's my data (questions follow):

SYNTHESIS
Mass of salicylic acid.., 2.0 g
Volume of acetic anhydride.., 5 mL
Mass of watch glass.., 22.10 g
Mass of aspirin and watch glass.., 28.01 g
Mass of aspirin.., 5.91 g

DETERMINATION OF PURITY
Mass of aspirin.., 0.34 g
Concentration of NaOH.., 0.1 M
Initial buret reading.., 0 mL
Final buret reading.., 15.8 mL
Volume of NaOH used.., 15.8 mL
Number of moles of NaOH required.., 1.60 x10^-3 mol

1. Calculate your mass percent yield of aspirin.

(actual yield, g/theoretical yield, g) x 100

Not sure what numbers I'd use here...

2. Calculate the percent purity of your aspirin.

(mass of aspirin, g (titration)/mass of aspirin, g (synthesis)) x 100

(0.34 g/5.91 g) x 100 = 5.8%

I'm pretty sure that's right, but not positive.


I'm not sure either but here is some discussion.
From synthesis:
Convert mass salicylic acid to mols.

Convert mols salicylic acid to mols aspirin know that 1 mol salicylic acid should produce 1 mol aspirin.

Convert mols aspirin to grams aspirin from mols aspirin x molar mass aspirin = g aspirin theoretical yield.

According to my calculations, 2.0 g salicylic acid should produce about 2.6 g aspirin. According to your data, you produced 5.91 g and that is WAY over what you should have obtained. Perhaps you didn't dry the aspirin on the watch glass very well before weighing.

If all of the synthesis part is as I understand it, then actual yield is 5.91 (I GUESS we are assuming all of that is aspirin) and theoretical yield is 2.61 and that makes for greater than 200% yield.

For #2, the answer is more straight forward. The only complaint I have here is that the 0.34g is labeled as mass aspirin when in reality it is the mass of the SAMPLE. From below you see mass of aspirin in that sample is less than that).
mols NaOH = M x L = 0.1 x 0.00158 = ??
mols aspirin = the same number.
multiply mols aspirin x molar mass aspirin to obtain grams aspirin. I found 0.284 g (that's out of the 0.34 g sample taken for analysis). That divided by 0.34 (that is the mass of sample taken for analysis) x 100 = %purity and I found about 84% purity. Of course the immediate question is, "What is rest of it?" Since you have over 200% yield, then we might expect SOME of the sample to be impure aspirin BUT there is a discrepancy even in these numbers for 84% yield means you have an extra 16% something else there but the theoretical yield is so much MORE than that. There is another 185% somewhere.
I suspect the original sample was not dried thoroughly AND I suspect that some time elapsed between the preparation of the sample and the determination of purity which gave it some time to air dry more.
I hope this gets you started.


Mass percent yield is found first by getting your experimental yield in grams by multiplying grams of SA sample used, times the molar mass of SA, times the mole ratio of SA and aspirin, times the molar mass of aspirin. Then divide the experimental (actual) yield by the theoretical yield, then multiply by 100 to get the mass percent yield.

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