Chemistry question(uegent)

Nasik Kwok

2008-06-01 12:06 am

Excess ethanedioic acid was added to 500cm3 of the calcium chloride solution.The precipitate of calcium ethandioate was filtered off and dissolved in dilute sulphuric(VI)acid.The resultant solution was diluted to 1.0dm3 and labelled "p".

25.00 cm3 solution 'P' was titrated againast a standard solution of 0.02M potassium manganate(VII).At the end point of the titration, the volume of manganate(VII) solution used is 17.8 cm3.

(a)Write equations to illustrate the reaction between
(i) calcium ethandioate and dilute sulphuric(VII)acid
(ii)solution'P' and potassium manganate(VII)

(b) What is meant by standard solution?
(c) Does this titration need an indicator? If an indicator is need, what is it? If not, briefly explain your answer.

回答 (1)

Uncle Michael

2008-06-01 1:22 am

✔ 最佳答案

(a)
(i)
CaC2O4 + H2SO4 → H2C2O4 + CaSO4

(ii)
2KMnO4 + 3H2SO4 + 5H2C2O4 → K2SO4 + 2MnSO4 + 8H2O + 10CO2
OR: 2MnO4- + 6H+ + 5H2C2O4 → 2Mn2+ + 8H2O + 10CO2
For convenience, the ionic equation may also written as follows:
2MnO4- + 16H+ + 5C2O42- → 2Mn2+ + 8H2O + 10CO2

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(b)
A standard solution is the solution with known concentration (of the solute).

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(c)
This titration does NOT need an indicator.
When KMnO4 solution is added to solution P, the purple KMnO4 solution is decolourized because the purple MnO4- ion is reduced to colourless Mn2+ ion by the ethanedioic acid in acidic solution P. At the equivalence point, all ethanedioic acid is completely reacted. A further very small amount of KMnO4 solution added would not react, but would change the solution to pale pink (or pale purple). This indicates the end point of the titration.

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