Chemistry 9701 · AS & A Level · Electrochemistry

Electrochemistry — practice question

Sodium chlorate(I), $\text{NaClO}$, is the active component in commercial bleach. The chlorate(I) ion concentration was found by titration. • $10.0\,\text{cm}^3$ of a bleach solution was made up to $250\,\text{cm}^3$ in a volumetric flask with distilled water. • Dilute sulfuric acid and an excess of potassium iodide solution were added to a $25.0\,\text{cm}^3$ portion of this solution to release iodine. • The resulting solution needed $20.80\,\text{cm}^3$ of $0.100\,\text{mol dm}^{-3}$ aqueous sodium thiosulfate solution to react with the iodine formed. The titration reactions are shown: $\text{ClO}^- + 2\text{I}^- + 2\text{H}^+ \rightarrow \text{I}_2 + \text{Cl}^- + \text{H}_2\text{O}$ $\text{I}_2 + 2\text{S}_2\text{O}_3^{2-} \rightarrow 2\text{I}^- + \text{S}_4\text{O}_6^{2-}$
(a)[3]

Calculate the concentration, in $\text{mol dm}^{-3}$, of $\text{ClO}^-$ ions in the bleach solution.

(b(i))[1]

Name a suitable indicator for this titration.

(b(ii))[1]

State the colour change you would expect to see at the end-point in this titration.

(b(iii))[1]

State at what stage in the procedure you would add the indicator.

(c)[2]

When an excess of aqueous hydrogen peroxide was added to $5.0\,\text{cm}^3$ of a different bleach solution, $82\,\text{cm}^3$ of oxygen was produced at room temperature and pressure. Calculate the concentration of $\text{ClO}^-$ ions in this bleach solution.

(d(i))[1]

Write the expression for $K_c$ for the equilibrium: $\text{C}_3\text{Cl}_3\text{N}_3\text{O}_3 + 3\text{H}_2\text{O} \rightleftharpoons \text{C}_3\text{H}_3\text{N}_3\text{O}_3 + 3\text{HClO}$

(d(ii))[2]

In outdoor swimming pools, $\text{HClO}$ is decomposed by sunlight. The decomposition of $\text{HClO}$ is a redox reaction which forms a gas that relights a glowing splint. Describe and explain the effect of the decomposition of $\text{HClO}$ on the equilibrium in (d). State the effect on $K_c$.

(d)

Trichlorocyanuric acid, $\text{C}_3\text{Cl}_3\text{N}_3\text{O}_3$, works as a chlorine buffer and disinfectant in swimming pools. It reacts with water to produce chloric(I) acid, $\text{HClO}$. $\text{C}_3\text{Cl}_3\text{N}_3\text{O}_3 + 3\text{H}_2\text{O} \rightleftharpoons \text{C}_3\text{H}_3\text{N}_3\text{O}_3 + 3\text{HClO}$

(d(iii))[1]

The decomposition of $\text{HClO}$ is a redox reaction. Suggest an equation for this reaction.

(e)

The buffer system in blood is a mixture of carbonic acid, $\text{H}_2\text{CO}_3$, and hydrogencarbonate ions, $\text{HCO}_3^-$. Healthy blood has a pH of $7.40$. $\text{H}_2\text{CO}_3 + \text{H}_2\text{O} \rightleftharpoons \text{HCO}_3^- + \text{H}_3\text{O}^+ \qquad K_a = 7.94 \times 10^{-7}\,\text{mol dm}^{-3}$

(e(i))[2]

Explain how this buffer system works to control the blood pH. Include equations in your answer.

(e(ii))[2]

A patient’s blood has a $[\text{HCO}_3^-] : [\text{H}_2\text{CO}_3]$ ratio of $9.5 : 1$. Calculate the pH of the patient’s blood.

Worked solution & mark scheme

This 16-mark question has a full step-by-step worked solution and mark scheme. One marking point: Correct working gives $[ClO^-]=1.04\,mol\,dm^{-3}$

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