Imagine that you have a pure sample of an unknown compound that you are told is either 2-naphthol OR benzoic acid. In the lab, you also have access to samples of pure 2- naphthol and pure benzoic acid. However, both of these pure substances melt at 123 °C: the same temperature. Briefly describe how you could determine the identity of your unknown sample using melting points ONLY. HINT: impure substances have different melting points than pure substances.

Question

I have no idea. I know that impure substances have lower melting points than pure substances, but that's all I know. How should I approach this problem? Thanks!

Answer 1

remember Molality? That determines the change in freezing point.

change freezing point=K*molessolute/moles solvent

First determine the freezing point depression for the solvents napthol, do this by experiment using a couple of solutes.
Then, knowing K, take the unknown let it be the solvent (add a known moles of one of the solvents you used in the first experiment, measure the new freezing point depression, then compute from the mass of the unknown solvent, how many moles you had, based on freezing pt depression. You know then if it is Napthaol or Benzoic acid.

Answer 2

That is great advice! Thanks, but the question asks specifically for using only melting point, so how do I do it with only melting point?