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February 7, 2016

# Homework Help: Chemistry

Posted by Katy on Thursday, September 29, 2011 at 1:18am.

I'm supposed to do this for an organic chemistry prelab. I've read my books and tried Googling everything possible, but I still don't know how to set this up. Please help!

The solubility of a certain compound in ice-cold water is 0.18g in 100 mL. Its solubility in hot water is 2.35g in 100 mL. What is the best possible percent recovery we can expect for recrystallization of this compound from water?
• Chemistry - DrBob222, Thursday, September 29, 2011 at 1:39am

You don't have enough information posted to work the problem. Here is what you do.
Look at your procedure and calculate the grams of product you expect to make assuming 100% for the reaction. Then multiply 0.18 x (volume you expect/100) and subtract from the product you expect to have. Finally, the amount left/amount expectedt)*100 = maximum recovery you can expect. You won't get that because the reaction won't be 100%. The idea is that if the solubility is 0.18 g/100 mL, you will lose that much per 100 mL during recrystallization (if just one recryst is done). If you expect to recrystallize more than once, you will lose that much each time.

• Chemistry - DrBob222, Thursday, September 29, 2011 at 3:39pm

I've had second thoughts about this. Although I think my initial response is correct, I believe we can approach it another way much simpler.
If we take a 2.35g sample, we can dissolve it in 100 mL water. If we then crystallize it at ice cold temperature, 0.18g of it will be lost. So the amount recovered is 2.35-0.18 =2.17g
%recovery = (2.17/2.35)*100 = x or about 92%