Determine the concentration of the magnesium and hydroxide ions in a saturated solution of magnesium hydroxide at 298 K, and calculate its pH. Assume there are no other ions present.

Deduce, with a reason, whether the pH of a saturated solution of aluminium hydroxide, at the same temperature, would be greater or less than your answer to (i).

\({K_{{\text{sp}}}} = {\text{[M}}{{\text{g}}^{2 + }}{\text{][O}}{{\text{H}}^ - }{{\text{]}}^2}\);

\(1.80 \times {10^{ - 11}} = (x){(2x)^2} = 4{x^3}/x = 1.65 \times {10^{ - 4}}\);

\({\text{[M}}{{\text{g}}^{2 + }}{\text{]}} = 1.65 \times {10^{ - 4}}{\text{ (mol}}\,{\text{d}}{{\text{m}}^{ - 3}}{\text{)}}\) and \({\text{[O}}{{\text{H}}^ - }{\text{]}} = 3.30 \times {10^{ - 4}}{\text{ (mol}}\,{\text{d}}{{\text{m}}^{ - 3}}{\text{)}}\);

\({\text{(pOH}} =  - \log 3.30 \times {10^{ - 4}} = 3.48{\text{ so) pH}} = (14 - 3.48 = ){\text{ }}10.5\);

Accept values from 10 to 11.

Do not penalize incorrect significant figures throughout.

Allow [2 max] for final correct pH if no working shown.

pH lower because lower concentration of \({\text{O}}{{\text{H}}^ - }\) ions / lower solubility/\({K_{{\text{sp}}}}\);

Must give reason to score mark.

Award mark for valid calculation.

Option E was one of the less popular options. In outlining the meaning of the term BOD, most candidates scored at least one mark out of two - typically the time and temperature condition - was not specified. Often in the reason why the concentration of dissolved oxygen falls, candidates did not identify aerobic respiration or decomposition of the organic matter by oxygen or that the increase in dissolved oxygen is from air. Graph for the effect of temperature on the concentration of dissolved oxygen was generally done well showing a line or a curve with a negative slope. Description of the physical and biological functions of SOM improve the quality of the soil was typically done well but answers revealed that the understanding of SOM is shallow with more sophisticated alternatives being rare.

In the question on \({K_{{\text{sp}}}}\) many candidates at least scored one mark from correctly stating the \({K_{{\text{sp}}}}\) expression with stronger candidates scoring fully. Deduction of the pH of aluminium hydroxide at the same pH and reduction of the toxicity of soil by increasing the pH shows a lack of understanding of key equilibrium and \({K_{{\text{sp}}}}\) concepts.

Although chemical equations that show the natural depletion of ozone were correct, some did not read the question carefully and stated the effect of CFCs on ozone depletion. Example of two ozone-depleting substances was well done; however, it is important to underline that often NO was presented rather than \({\text{N}}{{\text{O}}_x}\) or oxides of nitrogen. In the formation of ozone in smog, many candidates were able to state the reaction between the oxygen radical and oxygen molecule, but only a few candidates scored fully as a result of not starting from nitrogen(II) oxide as stated in the question.

Option E was one of the less popular options. In outlining the meaning of the term BOD, most candidates scored at least one mark out of two - typically the time and temperature condition - was not specified. Often in the reason why the concentration of dissolved oxygen falls, candidates did not identify aerobic respiration or decomposition of the organic matter by oxygen or that the increase in dissolved oxygen is from air. Graph for the effect of temperature on the concentration of dissolved oxygen was generally done well showing a line or a curve with a negative slope. Description of the physical and biological functions of SOM improve the quality of the soil was typically done well but answers revealed that the understanding of SOM is shallow with more sophisticated alternatives being rare.

In the question on \({K_{{\text{sp}}}}\) many candidates at least scored one mark from correctly stating the \({K_{{\text{sp}}}}\) expression with stronger candidates scoring fully. Deduction of the pH of aluminium hydroxide at the same pH and reduction of the toxicity of soil by increasing the pH shows a lack of understanding of key equilibrium and \({K_{{\text{sp}}}}\) concepts.

Although chemical equations that show the natural depletion of ozone were correct, some did not read the question carefully and stated the effect of CFCs on ozone depletion. Example of two ozone-depleting substances was well done; however, it is important to underline that often NO was presented rather than \({\text{N}}{{\text{O}}_x}\) or oxides of nitrogen. In the formation of ozone in smog, many candidates were able to state the reaction between the oxygen radical and oxygen molecule, but only a few candidates scored fully as a result of not starting from nitrogen(II) oxide as stated in the question.