Vapour Liquid Equilibria Questions and Answers

Answer: A

Correct answer: the first bubble of vapour appears

The bubble point is where a liquid heated at constant pressure just begins to boil, and the vapour in that first bubble is richer in the more volatile component than the liquid it came from. Its counterpart is the dew point, where the last drop of liquid disappears - or, coming the other way, where a cooled vapour first condenses. Between the two the mixture is two-phase.

Answer: B

The first bubble is richer in the more volatile component, which is exactly what makes distillation possible. Only at an azeotrope do the two compositions coincide, and that is why an azeotrope cannot be separated by simple distillation.

Answer: C

Reading a phase diagram is a matter of knowing which curve bounds which region and what a horizontal cut means. The tie line in particular is what the lever rule is applied to. Every diagram is drawn at one stated pressure.

Answer: D

It is the distribution ratio between the phases and is the working quantity in every flash calculation. For an ideal system it reduces to the vapour pressure divided by the total pressure, but in general it depends on composition too.

Answer: A

For an ideal system the relative volatility is the ratio of the pure component vapour pressures, which is one hundred and twenty over forty. Larger values mean an easier separation, and a value of one means none is possible.

Answer: B

Both are true and the reason gives the mechanism: distillation works only because the vapour is enriched, so when the enrichment vanishes no number of stages helps. The limit is thermodynamic rather than one of contacting.

Answer: C

It is the simplest useful correction for a non-ideal liquid at low pressure, where the vapour may still be treated as ideal. Adding a vapour fugacity coefficient gives the full gamma-phi formulation. Below a few bar the vapour correction is negligible.

Answer: D

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: positive deviations from Raoult’s law raise the total pressure and produce a minimum boiling azeotrope, and at the azeotropic point the vapour and liquid have the same composition - which is precisely why no further separation is possible by ordinary distillation. Statement 3 is wrong, and usefully so: the composition shifts with pressure, which is the basis of pressure-swing distillation.

Answer: A

Once the azeotropic composition is reached the vapour and liquid are identical and no further enrichment occurs. Anhydrous ethanol requires an entrainer, a membrane or a molecular sieve instead. The last few per cent of water is the expensive part.

Answer: B

Extra stages cannot help, because the limit is thermodynamic rather than one of contacting. The other three all change the equilibrium itself rather than approaching it more closely. Reflux and stages both address contacting, not equilibrium.