About A-level Chem past paper

我想問 2002 paper 1 question 7a(i)

我唔明白﹐點解當tin(II) sulphate is titrated against potassium

permanganate的時候,solution colour 會change from colouless to

pale pink(solution book同marking都係咁寫),正常唔係應該

change from purple (MnO4-) to coourless/pale pink(Mn2+)咩?

1 Answer

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  • 1 decade ago
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    When you describe the colour change at the end - point of a titration , you are referring to the colour change in the conical flask.

    In the above case, tin (II) sulphate is in the conical flask and it is colourless when titration starts.

    When a drop of acidified potassium permanganate is added to the conical flask, the pink droplet immediately turns colourless as MnO4- is reduced to Mn2+ .

    But then at the end - point of the titration, all the MnO4- ions have just been reduced to Mn2+ ions ( i.e. tin ( II ) sulphate has been totally consumed ).

    So when a further drop of MnO4- is added, a pink / pale pink colour is observed in the solution.

    Hence we would describe the colour change as from colourless to pale pink.

    Source(s): My Chem Knowledge
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