Question

3) Sketch a phase diagram (temperature vs. average system composition) of an ideal solution of two components A and B. Explain how you can obtain a pure sample of B from this mixture using fractional distillation. Indicate on your phase diagram the purification process you describe in your explanation.

Answer 1

Answer: We have taken two component A and B. Figure below shows the phase diagram which shows the liquid composition curve and vapor composition curve. As we can see in the diagram on left hand side we have pure B and on the right we have pure A. Suppose we boil a liquid mixture X₁, it will boil at temperature T₁ and the vapor over the top of the boiling liquid will have composition X₂. All we have to do is to use the liquid composition curve to find the boiling point of the liquid, and then look at what the vapour composition would be at that temperature. The vapour is much richer in the more volatile component B than the original liquid mixture was. Now suppose that we collected and condensed the vapour over the top of the boiling liquid and reboiled it. we now have a new liquid which had a composition X₂ that would boil at a new temperature T₂, and the vapour over the top of it would have a composition X₃. We can see that we now have a vapour which is getting quite close to being pure B. If we keep on doing this (condensing the vapour, and then reboiling the liquid produced) we will eventually get pure B. This whole procedure is the basis for fractional distillation.

Boiling point 4 pure A Vabor composition Boiling Point of 8 Liquid Composien OA X2 X Moleto action IOA OB HOB