Question

1. An experimental melting point is always reported as a range of temperatures (for example 171-174˚C). A student observed the following while taking a melting point: “The sample started shrinking at 93˚C. At 95˚C some liquid appeared. By 97˚C, the sample was a pool of liquid with a chunk of solid suspended in it. The suspended solid continued turning into liquid, becoming completely liquid at 98˚C”.

a)What should the student report as the melting point?

b) What two types of information about an unknown solid can be obtained from its experimental melting point?

c) Suppose that a pure unknown compound could be either compound A, compound B, or neither. The following information was obtained: The listed reference melting points are: compound A is 111-112˚C, compound B is 145-147˚C. The experimental melting for the unknown alone was 145-147˚C. The experimental melting point of a mixture of the pure unknown with pure B was 135-146˚C The compound is therefore: (pick one): Compound A, Compound B, neither A nor B Explain.

Answer 1

(a) The point or temperature from where any crystalline solid gives the first appearance to the liquid is its MELTING POINT. In the given sample, therefore 93°C is the melting point whereas 93-98°C is the melting range.

(b) From melting point, first we can assess the purity of the solid sample by comparing its observed m.p. range. And the other one, we can assure if the compound is pure or not. Since the soluble impurities can depress or broaden the melting point of pure compound.

(c) From the given information in part c, it has concluded that the mixed melting point is depressed to that of pure B, therefore the unknown compound is B because on adding the sample A, the mixed melting point is reduced/depressed to 135-136°C from the m.p. of pure B.