State the relationship between NH₄⁺ and NH₃ in terms of the Brønsted–Lowry theory.

Determine the concentration, in mol dm^–3, of the solution formed when 900.0 dm³ of NH₃(g) at 300.0 K and 100.0 kPa, is dissolved in water to form 2.00 dm³ of solution. Use sections 1 and 2 of the data booklet.

Calculate the concentration of hydroxide ions in an ammonia solution with pH = 9.3. Use sections 1 and 2 of the data booklet.

Calculate the concentration, in mol dm^–3, of ammonia molecules in the solution with pH = 9.3. Use section 21 of the data booklet.

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) $\rightleftharpoons$ 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.

Magnesium salts form slightly acidic solutions owing to equilibria such as:

Mg²⁺(aq) + H₂O (l) $\rightleftharpoons$ Mg(OH)⁺(aq) + H⁺(aq)

Comment on the role of Mg²⁺ in forming the Mg(OH)⁺ ion, in acid-base terms.

Mg(OH)⁺ is a complex ion, but Mg is not regarded as a transition metal. Contrast Mg with manganese, Mn, in terms of one characteristic chemical property of transition metals, other than complex ion formation.

conjugate «acid and base» ✔

amount of ammonia $⟨⟨=\frac{P.V}{R.T}=\frac{100.0 kPa\times 900.0 d{m}^3}{8.31 J K^{-1}mo{l}^{-1}\times 300.0 K}⟩⟩ = 36.1 «\mathrm{mol}»$ ✔

concentration $⟨⟨=\frac{n}{V}=\frac{36.1}{2.00}⟩⟩=18.1 «\mathrm{mol} {\mathrm{dm}}^{-3}»$ ✔

Award [2] for correct final answer.

$\left[{\mathrm{OH}}^{-}\right] ⟨⟨=\frac{{\mathrm{K}}_{\mathrm{W}}}{\left[{\mathrm{H}}^{+}\right]}=\frac{{10}^{-14}}{{10}^{-9.3}}={10}^{-4.7}⟩⟩=2.0 \times {10}^{-5}⟨⟨\mathrm{mol} {\mathrm{dm}}^{-3}⟩⟩$  ✔

$K_b=\frac{\left[N{H}_4^{+}\right]\left[O{H}^{-}\right]}{\left[N{H}_3\right]}/\frac{{10}^{-4.7}\times {10}^{-4.7}}{\left[N{H}_3\right]}⟨⟨={10}^{-4.75}⟩⟩$ ✔

$\left[{\mathrm{NH}}_3\right]=⟨⟨=\frac{{10}^{-9.4}}{{10}^{-4.75}}={10}^{-4.65}⟩⟩=2.24\times {10}^{-5}«\mathrm{mol} {\mathrm{dm}}^{-3}»$ ✔

Accept other methods of carrying out the calculation.

Award [2] for correct answer.

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».

Lewis acid ✔

accepts «a lone» electron pair «from the hydroxide ion» ✔

Do not accept electron acceptor without mention of electron pair.

ALTERNATIVE 1

Property: variable oxidation state ✔

Comparison: Mn compounds can exist in different valencies/oxidation states AND Mg has a valency/oxidation state of +2 in all its compounds ✔

Accept valency.
Accept for second statement “Mg «always» has the same oxidation state”.

ALTERNATIVE 2

Property: coloured ions/compounds/complexes ✔

Comparison: Mn ions/compounds/complexes coloured AND Mg ions/compounds white/«as solids»/colourless «in aqueous solution» ✔

Accept Mn forms coloured ions/compounds/complexes and Mg does not.

ALTERNATIVE 3

Property: catalytic activity ✔

Comparison: «many» Mn compounds act as catalysts AND Mg compounds do not «generally» catalyse reactions ✔


For any property accept a correct specific example, for example manganate(VII) is purple.
Do not accept differences in atomic structure, such as partially filled d sub-levels, but award ECF for a correct discussion.

Well done; However, instead of identifying the conjugate acid-base relationship, some simply identified these as Brønsted–Lowry base and acid.

Good performance. Some teachers suggested the question had an error in units, but this was not the case. The question had to be solved, first by using the data provided for application of gas law to determine the number of moles of gas. Next, given volume of solution, V = 2.00 dm³, determine its concentration.

Concentration of [OH^˗] was asked for but some calculated [H₃O⁺] instead. On the whole, question was done well.

Mediocre performance. Since a mark was given for the K_b expression, that mark could also be scored for the Henderson Hasselbalch (HH) equation, provided it is specific to the equilibrium reaction. Unfortunately, there was poor understanding of the application of the equation in most cases. Students should be strongly encouraged to use the HH equation only when a buffer is involved. Appropriate K_a or K_b expressions should be used when buffer solutions are not involved.

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⁺.

Mediocre performance; although some Mg²⁺ was identified as a Lewis acid, the reasoning given was that it accepts an electron, rather than an electron pair or references were made to Bronsted-Lowry theory.