Question 20N.3.hl.TZ0.18
| Date | November 2020 | Marks available | [Maximum mark: 4] | Reference code | 20N.3.hl.TZ0.18 |
| Level | hl | Paper | 3 | Time zone | TZ0 |
| Command term | Calculate, Describe | Question number | 18 | Adapted from | N/A |
A mixture of ethanal, ethanol and ethanoic acid is fractionally distilled.
Calculate the mole fraction of ethanal in the mixture.
[1]
✔
Accept “”.
This question involving Raoult's Law was very well answered and most were able to calculate the mole fraction of ethanal in the mixture (0.250) and the corresponding vapour pressure of ethanal above the liquid mixture at 20 °C (25.3 kPa). There was one G2 comment on this question. One teacher stated that the diagram shows four fractions but the stem of the question specifically states only three components and hence the fourth test tube is not required. The teacher commented that some students may have been distracted by this.
The vapour pressure of pure ethanal at is .
Calculate the vapour pressure of ethanal above the liquid mixture at .
[1]
✔
This question involving Raoult's Law was very well answered and most were able to calculate the mole fraction of ethanal in the mixture (0.250) and the corresponding vapour pressure of ethanal above the liquid mixture at 20 °C (25.3 kPa). There was one G2 comment on this question. One teacher stated that the diagram shows four fractions but the stem of the question specifically states only three components and hence the fourth test tube is not required. The teacher commented that some students may have been distracted by this.
Describe how this mixture is separated by fractional distillation.
[2]
Any two of:
continuous evaporation and condensation
OR
increased surface area in column helps condensation ✔
Accept “glass «beads» aid condensation «in fractionating column»”.
temperature decreases up the fractionating column ✔
liquids condense at different heights
OR
liquid of lowest boiling point collected first
OR
liquid with weakest intermolecular forces collected first
OR
most volatile component collected first
OR
fractions/liquids collected in order of boiling point/volatility ✔
Accept “liquids collected in order of molar mass”.
In this question candidates were required to describe how the mixture can be separated by fractional distillation. Only the better candidates scored both marks, though most gained at least one mark, usually for stating that the most volatile component is collected first. Many did not convey the idea that there is continuous evaporation and condensation in the process or the fact that the temperature decreases up the fractionating column.
