What are concordant titre volumes?

Titre volumes within ± 0.10 cm3 of each other.

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Explain why potassium permanganate (KMnO4) titrations are self-indicating.

During the titration, purple MnO4- is reduced and the solution becomes colourless. At the equivalence point, excess MnO4- causes the solution to turn purple again, indicating the end point without needing an additional indicator.

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2S2O32-(aq) ➔ S4O62-(aq) + 2e-

I2(aq) + 2e- ➔ 2I-(aq)

Combine the half-equations above to give the overall equation for the iodine-thiosulfate redox titration.

I2(aq) + 2S2O32-(aq) ➔ 2I-(aq) + S4O62-(aq)

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Why is starch indicator used in iodine-thiosulfate titrations?

Starch indicator forms a deep blue-black coluor with iodine, which helps to clearly identify the end point.

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Why is potassium iodide (KI) added in iodine-thiosulfate titrations?

Potassium iodide reacts with an oxidising agent through a redox reaction to produce iodine (I2) which is then available for the titration reaction. 

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How is the end point of an iodine-thiosulfate titration visualised when starch indicator is used?

The end point is detected when the blue-black colour of the starch-iodine complex disappears to give a colourless solution, indicating that all the iodine has reacted.

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What is the change in oxidation state of manganese during the Fe2+/MnO4- redox titration?

Manganese is reduced from +7 to +2


The half equation is: MnO4- + 8H+ + 5e- ➔ Mn2+ + 4H2O

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What is the change in oxidation state of iron  during the Fe2+/MnO4- redox titration?

Iron is oxidised from +2 to +3


The half equation is: Fe2+ ➔ Fe3+ + e-

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MnO4-(aq) + 8H+(aq) + 5e- ➔ Mn2+(aq) + 4H2O(l)

Fe2+(aq) ➔ Fe3+(aq) + e-

Combine the half-equations above to give the overall equation for the iron-manganate(VII) redox titration.

5Fe2+(aq) + MnO4-(aq) + 8H+(aq) ➔ 5Fe3+(aq) + Mn2+(aq) + 4H2O(l)

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What colour change occurs at the end point of the iron-manganate(VII) titration?

Colourless to purple

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Explain whether MnO4- ions act as oxidising agents or reducing agents.

MnO4- ions act as oxidising agents because they undergo reduction themselves, causing other substances to be oxidised.

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What is the purpose of adding dilute sulfuric acid in manganate(VII) titrations?

Dilute sulfuric acid is added to manganate(VII) titrations to provide the necessary H+ ions for the reduction of MnO4- ions to Mn2+ ions.


The half-equation for this reduction is: MnO4- + 8H+ + 5e- ➔ Mn2+ + 4H2O.

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