A Latimer diagram for a chemical element is a summary of the standard electrode potential data for that element. In a Latimer diagram, the form of the element with the highest oxidation state is on the left, with successively lower oxidation states to the right. A Latimer diagram for manganese at $$\(\mathrm{pH}=0\)$$ is shown. The diagram shows that the standard electrode potential for the reduction of $$\(\mathrm{MnO}_{4}^{-}\)$$to $$\(\mathrm{MnO}_{2}\)$$, in acidic conditions, is +1.69 V . $$\[ \mathrm{MnO}_{4}^{-}+4 \mathrm{H}^{+}+3 \mathrm{e}^{-} \rightleftharpoons \mathrm{MnO}_{2}+2 \mathrm{H}_{2} \mathrm{O} \quad E^{\ominus}=+1.69 \mathrm{~V} \]$$ Before use in titration experiments, potassium manganate(VII) solutions must be standardised. One method uses ethanedioate ions to find the exact concentration of the manganate(VII) ions. $$\(250.0 \mathrm{~cm}^{3}\)$$ of a standard solution contained 1.915 g of sodium ethanedioate, $$\(\mathrm{Na}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\)$$. A potassium manganate(VII) solution of approximately $$\(0.02 \mathrm{~mol} \mathrm{dm}^{-3}\)$$ was standardised using this solution. Excess sulfuric acid was added to $$\(25.0 \mathrm{~cm}^{3}\)$$ portions of the potassium manganate(VII) solution which were titrated with the sodium ethanedioate solution. The mean titre was $$\(22.95 \mathrm{~cm}^{3}\)$$. The relevant ionic half-equations are shown. $$\[ \begin{aligned} \mathrm{MnO}_{4}^{-}+8 \mathrm{H}^{+}+5 \mathrm{e}^{-} & \rightarrow \mathrm{Mn}^{2+}+4 \mathrm{H}_{2} \mathrm{O} \\ \mathrm{C}_{2} \mathrm{O}_{4}^{2-} & \rightarrow 2 \mathrm{CO}_{2}+2 \mathrm{e}^{-} \end{aligned} \]$$ (i) State the colour change at the end-point of the titration. (1) ................................................................................................................................................................................................................................................................................ ................................................................................................................................................................................................................................................................................ (ii) Calculate the accurate concentration of the potassium manganate(VII), in $$\(\mathrm{moldm}^{-3}\)$$, giving your answer to an appropriate number of significant figures. (4) (iii) A second titration carried out without the addition of sulfuric acid resulted in the formation of a brown suspension. Explain how the value of the mean titre would be affected, if at all, by the reaction that forms this suspension. Use the Data Booklet as a source of information. There is no need to calculate $$\(E_{\text {cell }}\)$$ values. (3)

Answer:

Knowledge points:

Solution: