[classicfan]

Hello.

I am having trouble with a question in my textbook.

If I have an 8.3 g sample of acetanilide (Solubility => 0.53g/100mL at 25oC and 5.0g/100 mL at 100oC) that needs to be purified by recrystallization, then what is the maximum % yield that can be recovered (10% excess of solvent was used)?

I'm confused as to how I go about such a question, so if you could, please, hint at what path I am supposed to take, I'd greatly appreciate it.

[Borek]

No matter how you will try, some of the acetanilide will be left in the solution.

[Heisenberg]

Is there any more information? Does it tell you how much solvent is used in the recrystallization or do you need to work that out?

The percentage recovery depends on the final temperature of the solution after recrystallization, as at 0 C the solute will most likely be less soluble than at 25 C. The question probably wants recovery at 25 C though considering they stated that temperature.

Heres what I've come up with:

Solubility at 100 C is equal to 1g per 20ml (100/5)

3.3 is the difference between 5g and 8.3g so to find out how much more solvent is needed than 100ml for 8.3g I did this:

100/5*3.3= 66(ml)

You would need to use 166ml of solvent to dissolve the solute at 100 C and a 10% excess to slow crystallization and aid purification would be 16.6ml, bringing the total to 182.6ml (183ml)

Cooling this down to 25 C would cause precipitation of acetanilide until 0.53g acetanilide is left per 100ml solvent, so that would leave 0.96778g of acetanilide in the solution (0.53*1.826).

Considering your solubility information is accurate to 2 decimal places the answer should be rounded to 2 decimal places, I think, so 0.97g left in solution.


8.3g - 0.97g = 7.33g (88% (7.33/8.3*100)) purified acetanilide recovered by cooling to 25 C.

I hope that's helped.

[mnakhla]

I agree with the above reply, but isn't some of the solvent generally allowed to evaporate off, wouldn't that need to be accounted for?

[Heisenberg]

I'm sorry, I didn't realise that it wasn't allowed to post full solutions in this forum.

I think to factor in solvent evaporation the identity of the solvent would have to be known, the vapor pressure or its volatility/evaporation rate determine how much solvent would evaporate in a given time at standard conditions, I think.

The question seems pretty vague and probably assumes 0% solvent evaporation to keep things simple.

[Heisenberg]

Actually, I think that the maximum number of decimal places that should be used in these calculations is 1 because the least accurately known measurement is to 1 decimal place, therefore using more than one decimal place is in fact no more accurate than using 1.

e.g. - 8.3g could be 8.39 or 8.30, .3 is accurately known but the second decimal place is not.

This means 0.5g should be used instead of 0.53, etc and the final answer may be slightly different.

[Borek]

No, 0.53 should be used. Rounding down initial values or intermediate resuolts is a common error. You should do calculations with full available precision.