F6 Chem TAS問題(2Q.)等住救命thx!!!!!

as mentioned, you've to show your attempt to the question.

1.
potassium dichromate is preferred over potassium permanganate, right?

permanganate itself in not very stable. it'll decompose slowly to give manganese dioxide and oxygen gas, especially under light. this unfavor the long storage of standard solution for keeping constant concentration over time.
(brown stains when permanganate comes into contact with clothes and skin).


2.
iron(II) tends to be oxidized by air under neutral or basic medium. according to wikiepdia:
4 Fe2+ + O2 + (4+2x) H2O ⇌ 2 Fe2O3.xH2O + 8 H+
oxidation of iron(II) is retarded by addition of protons, which shifts the equilibrium position to the left.
a little acid or acidic buffer can be added to stabilize the iron(II) solution. alternatively, iron(II) ammonium sulphate (ferrous ammonium sulfate) can be used to prepare the solutions, as ammonium ion would hydrolize and provide some protons.

also, please used freshly-prepared iron(II) solution.


as some iron(II) is oxidized, less dichromate is consumed, ;eading to under-estimation of concentration of iron(II).