[a confused chiral girl]

I read that it is necessary to use a condenser (in which gases are cooled into a liquid) during a recrystallization, but what are the advantages of using one?
Also, after we recover the product, how can the percent recovery during recrystallization be calculated?

thank you. 

[Custos]

In recrystallissation you are first trying to dissolve your compound in the minimum amount of boiling solvent right? So if the solvent is boiling away the volume is constantly reducing and hence your compound might start to come out of solution before you get to filter off insoluable impurities. One way to avoid this is to use a condensor, but this is usually only the case in industrial scale recrystallisation. In the lab we use a conical flask (rather than a beaker) because the shape acts to restrict the evaporation rate... and carry out the process quickly.

As for the percentage recovery -- have a guess yourself. You know what you start with, you can weight what you end with...

[movies]

I have never used a condenser to do recrystallization.  Usually you over-shoot the minimum amount of solvent you need anyway, so it's usually advantageous to boil off a little and saturate the solution.

[Custos]

Try recrystallising 30 kg of compound from 100 L of acetonitrile using that technique 

[a confused chiral girl]

my  little guess for percent recovery is...the mass of what I ended up with (purified) divided by the mass of the impure (what I started with) and then multiply by 100, right Custos? 

[Custos]

That's right. As an aside, I've seen students weigh their recovered product and get a higher mass (being still wet with solvent) than what they started with. They reported a greater than 100% recovery. Pretty cool trick if you could do it for real.

[movies]

Try recrystallising 30 kg of compound from 100 L of acetonitrile using that technique 

Ha!  Very true.  I don't think I've ever recrystallized more than maybe 50 g.