Question
a. Precipitation is one method used to treat waste water.
Phosphates, $\mathrm{PO}_4^{3-}$, in waste water can be removed by precipitation with magnesium ions. $K_{s p}$ of magnesium phosphate is $1.04 \times 10^{-24}$.
$$
3 \mathrm{Mg}^{2+}(\mathrm{aq})+2 \mathrm{PO}_4^{3-}(\mathrm{aq}) \rightarrow \mathrm{Mg}_3\left(\mathrm{PO}_4\right)_2(\mathrm{~s})
$$
Calculate the maximum solubility of phosphate ions in a solution containing $0.0100 \mathrm{~mol} \mathrm{dm}^{-3}$ magnesium ions.
b. Precipitation is one method used to treat waste water.
Zinc, cadmium, nickel, and lead are metal ions which can be removed by precipitation. Explain why waste water is adjusted to a pH of 9-10 to remove these ions by referring to section 32 of the data booklet.
▶️Answer/Explanation
Markscheme
a.
Accept “Ksp $=\left[\mathrm{Mg}^{2+}\right]^3\left[\mathrm{PO}_4{ }^{3-}\right]^2$ ” for M1.
Award [2] for correct final answer.
b. Any two of:
precipitation occurs with a base/carbonate $/ \mathrm{CO}_3{ }^{2-} /$ hydroxide/OH ${ }^{-}$
$\left[\mathrm{OH}^{-}\right]$is high enough to cause metal hydroxide precipitation at that $\mathrm{pH}$ these ions are slightly acidic/more soluble in acidic conditions only small amounts of carbonate/hydroxides/anion needed at that $\mathrm{pH}$ solubility products of the hydroxides are very small Do not accept “hydroxyl” for “hydroxide”.
Question
EDTA chelates with $\mathrm{Ni}^{2+}(\mathrm{aq})$.
$$
\left[\mathrm{Ni}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{2+}(\mathrm{aq})+\mathrm{EDTA}^{4-}(\mathrm{aq}) \rightleftharpoons\left[\mathrm{Ni}\left(\mathrm{EDTA}^{2-}\right]^{2-}(\mathrm{aq})+6 \mathrm{H}_2 \mathrm{O}(\mathrm{I})\right.
$$
a. Explain how entropy affects this equilibrium.
b. State the number of coordinate covalent bonds EDTA forms with $\mathrm{Ni}^{2+}$.
▶️Answer/Explanation
Markscheme
a. entropy increases «and the reaction proceeds to the right» [ $\boldsymbol{\sim}$ ]
more species / free molecules are formed
OR
more ways of distributing energy [ $\boldsymbol{U}]$
b. $\operatorname{six}[\circlearrowleft]$
Question
Heavy metals are toxic even in very low concentrations.
a. Outline why heavy metals are toxic.
b. Determine the maximum concentration of lead(II) ions at $298 \mathrm{~K}$ in a solution in which the concentration of carbonate ions is maintained at $1.10 \times$ $10^{-4} \mathrm{~mol} \mathrm{dm}^{-3}$. Use section 32 of the data booklet.
c. State a method, other than precipitation, of removing heavy metal ions from solution.
▶️Answer/Explanation
Markscheme
a. Any one of:
disrupt endocrine system
OR
compete for active sites of enzymes/cellular receptors
OR
form complexes with/inhibit enzymes
OR
denature proteins
OR
change shape of active site $[\boldsymbol{V}]$
臣임
participate in redox reactions
OR
disturb normal redox balance «in cells» $[\boldsymbol{}]$
initiate «free» radical reactions «in electron transfer» $[\boldsymbol{V}]$
b.
$$
\begin{aligned}
& « K_{s p}=7.40 \times 10^{-14} » \\
& K_{s p}=\left[\mathrm{Pb}^{2+}\right]\left[\mathrm{CO}_3^{2-}\right][\leftarrow] \\
& {\left[\mathrm{Pb}^{2+}\right] «=\frac{7.40 \times 10^{-14}}{1.10 \times 10^{-4}} »=6.73 \times 10^{-10} « \mathrm{~mol} \mathrm{dm}{ }^{-3} »[\longleftarrow]}
\end{aligned}
$$
Note: Award [2] for correct final answer.
c. Any one of:
chelation «by EDTA/polydentate ligand anchored» [ $/$ ion exchange systems [ $\boldsymbol{\sim}]$ adsorption by «water» plants $[\boldsymbol{\sim}]$
Note: Accept “use of zeolites”.