[AuFinger]

I'm taking organic chemistry after not having studied chemistry for about 6 years -- I'm about to do a lab in which I'll purify some sulfanilamide through crystallization.  I need to come up with the theoretical yield, and I'm not sure I remember how to do it correctly.  I'm starting with .3 grams of sulfanilamide (which actually contains the impurity of 5% acetanilide).  Since I'm just dissolving and then crystallizing out the sulfanilamide, aren't the reaction and product the same in this reaction?  If this is the case, shouldn't the experimental and stoichiometric ratios be 1, and my theoretical yield just be .3g?

[Borek]

Yes.

[DrCMS]

No of course not.

If you have 100 fruit and 5% are apples and the rest oranges can you really pick out 100 oranges? 
The most oranges you can possibly get are 95.  If you squash 5 oranges while separating the apples from the oranges then your yield is 90 oranges but the % yield is 90/95 = 94.7%

[Borek]

Sorry, somehow I have misread the question. If you know for sure that there is exactly 5% of impurities, you should follow DrCMS post.