Question

I did a lab about melting point. It is asking me for the purity of the compounds?

How do I find it?
Do I use melting points or percent yield?

Answer 1

How sharp the melting point is and how close to the accepeted melting point the value you find are indicators of how pure the sample might be. Thus a sharp melting point that agrees with the accepted value would indicate a high purity sample.
Even better would be to have a pure sample and the test sample together in the same apparatus, and if these melt at the same time then this is a good indicator that the test sample is pure.

Percentage yield is not a good indcator of purity. I have carried out many a reaction where the yield was low, say 30%, but the product was pure.