Question

1. You are presented with three vials each contains a white fluffy powder. You label the vials A, B, and C, then measure the melting point of each. The following is the data you collect for your observations: A: 80-82°C B: 113-114°C C: 80-82°C a) Is it safe to say A = C? What simple test can verify or refute this statement? (The correct answer must include how you would interpret the observations to conclude both the A = C and A ≠ C) b) Let’s assume A ≠ C, What simple test would indicate the presence of an impurity of a different polarity than the more abundant compound? What would you expect to observe if A was a pure compound and C was slightly contaminated with a soluble impurity? c) After C is purified, the melting point of the dry crystals is 113-114°C. If C = B then what would be the result of mixing the B & C together and measuring the melting point of the mixture?

Answer 1

1. For the given samples A, B and C

a) It is safe to say that A = C, based on melting point data.

An IR or 1H NMR of the samples can confirm the results. Both A and C would have the same finger print region in IR and same proton numbers and splitting pattern in NMR.

b) If A is not same as C. A thin layer chromatoraphic run of sample should yield different spots on plate of compound and impurity present.

In A we would see only one spot of tlc plate, whereas, in case of C we would see more than 1 spot of the tlc plate.

c) After purification, C melting point matched that of B. If B = C and we mix A and B, their should not be any change to the melting point of the system. melting point of B = C = B + C.