Phosphate ions can be removed from a solution by adding calcium ions. State the ionic equation for the reaction of calcium ions with phosphate ions.

Deduce the expression for the solubility product constant, \({K_{{\text{sp}}}}\), of calcium phosphate.

The solubility product of calcium phosphate is \({\text{2.07}} \times {\text{1}}{{\text{0}}^{ - 33}}\) at 298 K. Determine the concentration, in \({\text{mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\), of calcium ions, \({\text{C}}{{\text{a}}^{2 + }}\), in a saturated aqueous solution of calcium phosphate.

\({\text{3C}}{{\text{a}}^{{\text{2 + }}}}{\text{(aq)}} + {\text{2PO}}_{\text{4}}^{3 - }{\text{(aq)}} \rightleftharpoons {\text{C}}{{\text{a}}_{\text{3}}}{{\text{(P}}{{\text{O}}_{\text{4}}}{\text{)}}_{\text{2}}}{\text{(s)}}\);

Ignore state symbols.

Accept single arrow sign.

\({\text{(}}{K_{{\text{sp}}}} = {\text{) [C}}{{\text{a}}^{2 + }}{{\text{]}}^3}{{\text{[PO}}_4^{3 - }{\text{]}}^2}\);

Ignore state symbols.

Do not award mark if incorrect brackets are used or are missing.

Let x be solubility so \(2.07 \times {10^{ - 33}} = {(3x)^3}{(2x)^2}\);

Remember to apply ECF from (ii).

\({x^5} = \frac{{2.07 \times {{10}^{ - 33}}}}{{(27 \times 4)}}/1.92 \times {10^{ - 35}}/x = 1.14 \times {10^{ - 7}}\);

\(\left( {[{\text{C}}{{\text{a}}^{2 + }}] = 3x = } \right){\text{ }}3.42 \times {10^{ - 7}}{\text{ (mol}}\,{\text{d}}{{\text{m}}^{ - 3}}{\text{)}}\);

Award [3] for final correct answer.

Q was only identified by the stronger candidates and even then few stated that waste needs oxygen to decompose. The ionic equation for the reaction of calcium ions with phosphate ions proved a real minefield. It was highly disappointing at HL that so many candidates did not know what the formula and charge of the phosphate anion actually is. Some gave phosphite and several gave phosphide. Core chemistry underpins all options and candidates need to be prepared to apply some basic chemical principles to the various topics in the options. This aspect will be further enhanced in the new chemistry syllabus, but performance of candidates in this particular question shows the importance of this even in the current syllabus. In (ii), candidates often used the ionic equation in (i) to write the solubility product expression and hence had an incorrect inverse equation. Many also did not realize that the activity of a species in the solid state is unity. In (iii), incorrect \({K_{{\text{sp}}}}\) expressions in (ii) threw some candidates and others could not deal with the math involved in the solution to the equation. Many thought the final answer was x and not 3x for \({\text{[C}}{{\text{a}}^{2 + }}{\text{]}}\). Of course the stronger candidates scored all three marks on this question. In (c), misreading of the question was common which specifically asked for a non-chemical reason for the decrease in oxygen concentration i.e. an increase in the temperature of the water.

Q was only identified by the stronger candidates and even then few stated that waste needs oxygen to decompose. The ionic equation for the reaction of calcium ions with phosphate ions proved a real minefield. It was highly disappointing at HL that so many candidates did not know what the formula and charge of the phosphate anion actually is. Some gave phosphite and several gave phosphide. Core chemistry underpins all options and candidates need to be prepared to apply some basic chemical principles to the various topics in the options. This aspect will be further enhanced in the new chemistry syllabus, but performance of candidates in this particular question shows the importance of this even in the current syllabus. In (ii), candidates often used the ionic equation in (i) to write the solubility product expression and hence had an incorrect inverse equation. Many also did not realize that the activity of a species in the solid state is unity. In (iii), incorrect \({K_{{\text{sp}}}}\) expressions in (ii) threw some candidates and others could not deal with the math involved in the solution to the equation. Many thought the final answer was x and not 3x for \({\text{[C}}{{\text{a}}^{2 + }}{\text{]}}\). Of course the stronger candidates scored all three marks on this question. In (c), misreading of the question was common which specifically asked for a non-chemical reason for the decrease in oxygen concentration i.e. an increase in the temperature of the water.

Q was only identified by the stronger candidates and even then few stated that waste needs oxygen to decompose. The ionic equation for the reaction of calcium ions with phosphate ions proved a real minefield. It was highly disappointing at HL that so many candidates did not know what the formula and charge of the phosphate anion actually is. Some gave phosphite and several gave phosphide. Core chemistry underpins all options and candidates need to be prepared to apply some basic chemical principles to the various topics in the options. This aspect will be further enhanced in the new chemistry syllabus, but performance of candidates in this particular question shows the importance of this even in the current syllabus. In (ii), candidates often used the ionic equation in (i) to write the solubility product expression and hence had an incorrect inverse equation. Many also did not realize that the activity of a species in the solid state is unity. In (iii), incorrect \({K_{{\text{sp}}}}\) expressions in (ii) threw some candidates and others could not deal with the math involved in the solution to the equation. Many thought the final answer was x and not 3x for \({\text{[C}}{{\text{a}}^{2 + }}{\text{]}}\). Of course the stronger candidates scored all three marks on this question. In (c), misreading of the question was common which specifically asked for a non-chemical reason for the decrease in oxygen concentration i.e. an increase in the temperature of the water.