DP Chemistry (last assessment 2024)

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Question 22M.2.hl.TZ1.4c(iii)

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Date May 2022 Marks available [Maximum mark: 2] Reference code 22M.2.hl.TZ1.4c(iii)
Level hl Paper 2 Time zone TZ1
Command term Outline Question number c(iii) Adapted from N/A
c(iii).
[Maximum mark: 2]
22M.2.hl.TZ1.4c(iii)

Ammonia is soluble in water and forms an alkaline solution:

NH3 (g) + H2O (l) ⇌ NH4+ (aq) + HO– (aq)

(c(iii))

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]

Markscheme

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».

Examiners report

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+.

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