Another weak base is nitrogen trifluoride, ${\text{N}}{{\text{F}}_{\text{3}}}$. Explain how ${\text{N}}{{\text{F}}_{\text{3}}}$ is able to function as a Lewis base.

Calculate the pH of a ${\text{1.00}} \times {\text{1}}{{\text{0}}^{ - 2}}{\text{ mol}}\,{\text{d}}{{\text{m}}^{ - 3}}$ aqueous solution of ammonia at 298 K.

${\text{25.0 c}}{{\text{m}}^{\text{3}}}$ of $1.00 \times {10^{ - 2}}{\text{ mol}}\,{\text{d}}{{\text{m}}^{ - 3}}$ hydrochloric acid solution is added to ${\text{50.0 c}}{{\text{m}}^{\text{3}}}$ of ${\text{1.00}} \times {\text{1}}{{\text{0}}^{ - 2}}{\text{ mol}}\,{\text{d}}{{\text{m}}^{ - 3}}$ aqueous ammonia solution. Calculate the concentrations of both ammonia and ammonium ions in the resulting solution and hence determine the pH of the solution.

State what is meant by a buffer solution and explain how the solution in (v), which contains ammonium chloride dissolved in aqueous ammonia, can function as a buffer solution.

State the equations for the reactions of sodium oxide, ${\text{N}}{{\text{a}}_{\text{2}}}{\text{O}}$, and phosphorus(V)oxide, ${{\text{P}}_{\text{4}}}{{\text{O}}_{{\text{10}}}}$, with water.

it can donate the lone/non-bonding pair of electrons (on the N atom);

${K_{\text{b}}} = \frac{{{{{\text{[O}}{{\text{H}}^ - }{\text{]}}}^{\text{2}}}}}{{{\text{[N}}{{\text{H}}_{\text{3}}}{\text{]}}}} = {10^{ - 4.75}}/1.78 \times {10^{ - 5}}$;

${\text{[O}}{{\text{H}}^ - }{\text{]}} = \sqrt {(1.00 \times {{10}^{ - 2}} \times {{10}^{ - 4.75}})}  = 4.22 \times {10^{ - 4}}{\text{ (mol}}\,{\text{d}}{{\text{m}}^{ - 3}}{\text{)}}$;

${\text{pOH}} =  - {\log _{10}}(4.22 \times {10^{ - 4}}) = 3.37/[{{\text{H}}^ + }] = \frac{{1.00 \times {{10}^{ - 14}}}}{{4.22 \times {{10}^{ - 4}}}} = 2.37 \times {10^{ - 11}}$;

${\text{pH}} = 14 - 3.37 = 10.6$;

Award [2 max] for correct final answer if no working shown.

initial amount of ${\text{HCl}} = \frac{{25.0}}{{1000}} \times 1.00 \times {10^{ - 2}} = 2.50 \times {10^{ - 4}}{\text{ mol}}$ and initial amount of ${\text{N}}{{\text{H}}_3}{\text{ = }}\frac{{50.0}}{{1000}} \times 1.00 \times {10^{ - 2}}{\text{ = }}5.00 \times {10^{ - 4}}{\text{ mol}}$;

final amount of ${\text{NH}}_4^ +$ and ${\text{N}}{{\text{H}}_3}$ both $= 2.50 \times {10^{ - 4}}{\text{ mol}}$;

final ${\text{[NH}}_4^ + {\text{]}}$ and ${\text{[N}}{{\text{H}}_3}{\text{]}}$ both $= \frac{{2.50 \times {{10}^{ - 4}}}}{{75.0 \times {{10}^{ - 3}}}} = 3.33 \times {10^{ - 3}}{\text{ mol}}\,{\text{d}}{{\text{m}}^{ - 3}}$;

${\text{[O}}{{\text{H}}^ - }{\text{]}} = {K_{\text{b}}} \times \frac{{{\text{[N}}{{\text{H}}_{\text{3}}}{\text{]}}}}{{{\text{[NH}}_4^ + {\text{]}}}} = {K_{\text{b}}} = {10^{ - 4.75}}/1.78 \times {10^{ - 5}}$;

${\text{pOH}} = 4.75$ hence ${\text{pH}} = 9.25$;

Award final two marking points if half-equivalence method used.

a buffer solution resists a change in pH when small amounts of acid or base are added to it;

Do not accept description in terms of composition of buffer.

when ${{\text{H}}^ + }$ is added it reacts with ${\text{N}}{{\text{H}}_3}$ to form ${\text{NH}}_4^ +$;

when ${\text{O}}{{\text{H}}^ - }$ is added it reacts with ${\text{NH}}_4^ +$ to form ${\text{N}}{{\text{H}}_3}$ and ${{\text{H}}_2}{\text{O}}$;

Accept equations for last two marking points.

${\text{N}}{{\text{a}}_2}{\text{O}} + {{\text{H}}_2}{\text{O}} \to {\text{2N}}{{\text{a}}^ + } + {\text{2O}}{{\text{H}}^ - }/{\text{N}}{{\text{a}}_2}{\text{O}} + {{\text{H}}_2}{\text{O}} \to {\text{2NaOH}}$;

${{\text{P}}_4}{{\text{O}}_{10}} + {\text{6}}{{\text{H}}_2}{\text{O}} \to {\text{4}}{{\text{H}}_3}{\text{P}}{{\text{O}}_4}$;

Ignore state symbols.

In (a) (iii) some candidate did not mention the need for a lone pair even though they had an understanding of the need for a pair of electrons when explaining the basic properties of nitrogen trifluoride.

Answers to (a) (iv) were encouraging with many candidates able to calculate the pH from the ${\text{p}}{K_{\text{b}}}$ value for ammonia.

The more difficult (a) (v) was only answered correctly by the strongest candidates and a significant number left it blank.

Some candidates lost marks in (a) (vi) as they did not explicitly state that buffers are resistant to changes of pH when small amounts of acid or base are added. Many also did not respond directly to the requirements of the question and explain the action of the specific buffer mixture of ammonia and ammonium chloride. Salt hydrolysis was poorly understood.

Most could give an equation for the reaction of sodium oxide with water but the formation of phosphoric (V) acid from phosphorus (V) oxide proved more problematic.