Recrystallization Percent Recovery

[SciFiGuy2012]

Homework Statement

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?

Homework Equations

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]

Homework Statement

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?

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

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.

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.

 

[Blueshift5]

The percent recovery in recrystallization is a measure of the amount of pure solid that was successfully recovered from the impure sample. It is calculated by dividing the mass of the recovered pure solid by the initial mass of the impure sample and multiplying by 100%. This value does not necessarily represent the "true" value of the recovered solid, as it is possible for some impurities to remain in the final product. However, a higher percent recovery generally indicates a more successful recrystallization process.

As for the second question, there are a few ways to determine if the quality of the unknown solid has improved as a result of recrystallization. One way is to perform a melting point determination before and after recrystallization. If the melting point of the final product is higher and more narrow, it is an indication that the recrystallization process was successful in purifying the solid. Another way is to perform a qualitative analysis, such as a TLC or HPLC, to compare the impurities present in the initial sample to those in the final product. If the impurities are significantly reduced or eliminated, it is further evidence of the improved quality of the solid after recrystallization.