Chemistry 9701 · AS & A Level · Chemistry of transition elements

Chemistry of transition elements — practice question

An aqueous cobalt(II) chloride solution appears pink because it contains $[\text{Co}(\text{H}_2\text{O})_6]^{2+}$ complex ions.
(a(i))[2]

Complete Table 6.1 to show the observations seen when the named reagent is added to aqueous cobalt(II) chloride.

(a(ii))[1]

Write an equation for the reaction of $[\text{Co}(\text{H}_2\text{O})_6]^{2+}$ ions with $\text{NaOH(aq)}$.

(a(iii))[1]

Write an equation for the reaction of $[\text{Co}(\text{H}_2\text{O})_6]^{2+}$ ions with an excess of concentrated $\text{HCl}$.

(b(i))[1]

Define stability constant.

(b(ii))[1]

Write an expression for the stability constant, $K_{\text{stab}}$, of the $[\text{Co(NH}_3)_6]^{2+}$ complex ion.

(b(iii))[1]

State the units of the stability constant, $K_{\text{stab}}$, for the $[\text{Co(NH}_3)_6]^{2+}$ complex ion.

(b(iv))[1]

The numerical value of the stability constant, $K_{\text{stab}}$, for the $[\text{Co(NH}_3)_6]^{2+}$ complex ion is $7.7 \times 10^4$. In an aqueous solution, the concentration of the $[\text{Co(NH}_3)_6]^{2+}$ complex ion is $0.0740\text{ mol dm}^{-3}$ and the concentration of $\text{NH}_3$ is $0.480\text{ mol dm}^{-3}$ at equilibrium. Calculate the concentration of $[\text{Co}(\text{H}_2\text{O})_6]^{2+}$ in this solution.

Worked solution & mark scheme

This 8-mark question has a full step-by-step worked solution and mark scheme. One marking point: Blue or pink precipitate; blue solution

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