Question 22M.2.hl.TZ1.c(iii)
| Date | May 2022 | Marks available | [Maximum mark: 2] | Reference code | 22M.2.hl.TZ1.c(iii) |
| Level | hl | Paper | 2 | Time zone | TZ1 |
| Command term | Outline | Question number | c(iii) | Adapted from | N/A |
Ammonia is soluble in water and forms an alkaline solution:
NH3 (g) + H2O (l) NH4+ (aq) + HO– (aq)
An aqueous solution containing high concentrations of both NH3 and NH4+ acts as an acid-base buffer solution as a result of the equilibrium:
NH3 (aq) + H+ (aq) NH4+ (aq)
Referring to this equilibrium, outline why adding a small volume of strong acid would leave the pH of the buffer solution almost unchanged.
[2]
equilibrium shifts to right/H+ reacts with NH3 ✔
«as large excess» ratio [NH3]:[NH4+] «and hence pH» almost unchanged ✔
Accept “strong acid/H+ converted to a weak acid/NH4+ «and hence pH almost unchanged».
Mediocre performance. One mark was scored for suggesting equilibrium shifts to right or H+ reacts with NH3. However, some made reference to ammonia being a strong base or no reference to the strong acid, H+ being converted to a weak acid, NH4+.
