CAIE A-Level Chemistry 28.2.9 Transition-Metal Redox Calculations

Practise equations and titration calculations for acidified MnO4− with Fe2+ or C2O4²− and for Cu2+ with I−.

Exam points

use the 1:5 MnO4−:Fe2+ and 2:5 MnO4−:C2O4²− ratios after calculating titrant moles
scale aliquot moles to the original sample before finding iron mass, concentration or water of crystallisation
write balanced ionic equations and use the first permanent pale-pink colour as the MnO4− titration end-point

[Maximum number: 2]

Iodine is found naturally in compounds in many different oxidation states.

The concentration of $\mathrm{Cu}^{2+}(\mathrm{aq})$ in a solution can be determined by the reaction of $\mathrm{Cu}^{2+}$ ions with $\mathrm{I}^{-}$ ions.

$reaction $12 \mathrm{Cu}^{2+}+4 \mathrm{I}^{-} \rightarrow 2 \mathrm{CuI}+\mathrm{I}_{2}$$

The $\mathrm{I}_{2}$ produced in reaction 1 is titrated against a solution containing thiosulfate ions, $\mathrm{S}_{2} \mathrm{O}_{3}{ }^{2-}$ , using a suitable indicator.

\text { reaction } 2 \quad 2 \mathrm{~S}_{2} \mathrm{O}_{3}^{2-}+\mathrm{I}_{2} \rightarrow \mathrm{~S}_{4} \mathrm{O}_{6}^{2-}+2 \mathrm{I}^{-}

A $25.0 \mathrm{~cm}^{3}$ portion of a $\mathrm{Cu}^{2+}(\mathrm{aq})$ solution reacts with an excess of $\mathrm{I}^{-}(\mathrm{aq})$ .

The end-point of the titration occurs when $22.30 \mathrm{~cm}^{3}$ of $0.150 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{~S}_{2} \mathrm{O}_{3}{ }^{2-}(\mathrm{aq})$ is added.
Calculate the concentration of $\mathrm{Cu}^{2+}(\mathrm{aq})$ in the original solution.

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28.2.9—Transition metal redox calculations question 1