Question 20N.3.HL.TZ0.a.ii
| Date | November 2020 | Marks available | [Maximum mark: 2] | Reference code | 20N.3.HL.TZ0.a.ii |
| Level | HL | Paper | 3 | Time zone | TZ0 |
| Command term | Explain | Question number | a.ii | Adapted from | N/A |
Citric acid was produced in a fermenter using the pulp of cassava (Manihot esculenta) and the fungus Aspergillus niger. This process was carried out at room temperature for 6 days and citric acid was then collected. The apparatus was cleaned and set up for a new fermentation. The process is shown in the diagram.
[Source: Prado, F.C., Vandenberghe, L.P.S., Woiciechowski, A.L., Rodrígues-León, J.A. and Soccol,
C.R., 2005. Citric acid production by solid-state fermentation on a semi-pilot scale using different percentages of
treated cassava bagasse. Brazilian Journal of Chemical Engineering, 22(4), pp.547–555.]
Explain the possible causes of these changes in pH.
[2]
a. pH decreased by citric acid;
b. (pH lowered by) production of CO2 produced (by the fungus) in respiration;
c. (pH lowered by) production of carbonic acid by CO2 dissolving in water;
Most stated that the pH changes were due to acidic conditions, but fewer could explain how the acidic conditions were generated.
