[Juners]

I completed a lab that analyzed an antacid tablet.

The tablet was crushed into a powder, and 50ml of a known molarity HCl was added to the tablet. The HCl was in excess. The tablet was analyzed using a back titration method. Methyl purple was added to the tablet and acid and the solution turned purple indicating it was very acidic (due to the excess HCl).

A back titration analysis was completed by pipetting a known concentration of NaOH until the solution changed to a green color. The number of moles of HCl neutralized was calculated, and by basic stoichiometry, the number of mg of calcium carbonate (active ingredient) was calculated.

The tablet had an active ingredient of calcium carbonate of 500mg. However at the end of the titration, the calculations showed there to be close to 600mg of calcium carbonate meaning there was a percent yield of 120%.

How is this possible? The only thing I have come across is that there are impurities, however I do not know what this means....?

[Mitch]

That is not percent yield! It only means that there was 600mg, the manufacturer of the tablet mislabeled.

[Juners]

The manufacturer of the tablet mislabelled the the bottle?

That doesn't make sense to me, considering it was a national brand antacid... it doesnt seem right that it could just be mislabeled.

There should have been a yield of 100% meaning all calcium carbonate was accounted for. However there was a yield of 120% meaning too much was accounted for, which obviously doesn't make sense.

How is that not percent yield?

500mg were supposed to come out, but 560 did. Percent yield is actual yield divided by theoretical yield, multiplied by one hundred.
(560/500)x100=112%

[Yggdrasil]

The manufacturer of the tablet mislabelled the the bottle?

That doesn't make sense to me, considering it was a national brand antacid... it doesnt seem right that it could just be mislabeled.

There should have been a yield of 100% meaning all calcium carbonate was accounted for. However there was a yield of 120% meaning too much was accounted for, which obviously doesn't make sense.

How is that not percent yield?

500mg were supposed to come out, but 560 did. Percent yield is actual yield divided by theoretical yield, multiplied by one hundred.
(560/500)x100=112%

Companies will take many shortcuts to make their product cheaper.  Likely, the amount of antacid per tablet is not that important in terms of function or safety so the company probably produces tablets with 500mg +/- 100mg or so.  After all, as far as you know the 500mg value has only one significant figure.

Also, when you talk about yield you imply that you performed some sort of synthesis or purification.  A titration is a purely analytical technique which involves no purification or synthesis.

[Mitch]

If a company lists 500mg calcium carbonate and a lawsuit hungry individual sues them because they discovered it only contained 499mg. You better believe that company will error on the safe side.

[Borek]

How precise is your determination? 12% seems a lot, but it is not that difficult to make such an error.

[mike]

How much did the tablet weigh?

Could there be other alkaline components to the tablet? I don't think your analysis would diferentiate different alkalis.

Or, like Borek said, it could be precision problem.

[movies]

Do you know the purity of the tablet you started with?  It's possible there were other additives in there that were also basic.