Recrystallization Percent Recovery

SciFiGuy2012

1. The problem statement, all variables and given/known data
Calculating percent recovery of an unknown solid and describing if the percent recovery represents the "true" value of the recovered pure solid.

How do we know that quality of an unknown solid which was recrystallized improved as a result of the recrystallization?

2. Relevant equations


3. The attempt at a solution

For the first question, I am confused because I don't understand what the "true value" means. I got 75% as my percent recovery, but I don't understand what the question means by "true" value.

For the second question I have no idea what it's saying. After recrystallization we expect the compound to be purer. But we have to give some "evidence". So please help me there.

Please help.

Thanks.

chemisttree

You have to test it some way. What types of tests are you familiar with that deal with purity?

It means just what it implies. If you recover a solid from a fairly pure sample, the yield you get and the true recovery are one and the same. If the sample is heavily contaminated with something else, what you obtain might be all there is of the component that you recrystallize. In this case the true yield might be very good even though the percent recovery only reflects the (much lower?) concentration of the recrystallized material in the original sample.