Chemistry AQA titration question!

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bazinga12
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#1
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#1
A student weighed out a 2.29 g sample of impure K3[Fe(C2O4)3].3H2O and dissolved it
in water.
This solution was added to a 250 cm3 volumetric flask and made up to 250 cm3 with
distilled water.
A 25.0 cm3 portion was pipetted into a conical flask and an excess of acid was added.
The mixture was heated to 60 ºC and titrated with 0.0200 mol dm–3 KMnO4 solution.
26.40 cm3 of KMnO4 solution were needed for a complete reaction.
In this titration only the C2O4 2– ions react with the KMnO4 solution.

Question: Write an equation for the reaction between C2O4 2– ions and MnO4– ions in acidic
solution.
Calculate the percentage purity of the original sample of K3[Fe(C2O4)3].3H2O
Give your answer to 3 significant figures. (7marks)

I get all the steps apart from when determining the moles of K3[Fe(C2O4)3].3H2O
In the mark scheme moles of C2O4 in the original solution is being divided by 3 to find out its moles. Why?
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ChemistryWebsite
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#2
Report 3 years ago
#2
(Original post by bazinga12)
A student weighed out a 2.29 g sample of impure K3[Fe(C2O4)3].3H2O and dissolved it
in water.
This solution was added to a 250 cm3 volumetric flask and made up to 250 cm3 with
distilled water.
A 25.0 cm3 portion was pipetted into a conical flask and an excess of acid was added.
The mixture was heated to 60 ºC and titrated with 0.0200 mol dm–3 KMnO4 solution.
26.40 cm3 of KMnO4 solution were needed for a complete reaction.
In this titration only the C2O4 2– ions react with the KMnO4 solution.

Question: Write an equation for the reaction between C2O4 2– ions and MnO4– ions in acidic
solution.
Calculate the percentage purity of the original sample of K3[Fe(C2O4)3].3H2O
Give your answer to 3 significant figures. (7marks)

I get all the steps apart from when determining the moles of K3[Fe(C2O4)3].3H2O
In the mark scheme moles of C2O4 in the original solution is being divided by 3 to find out its moles. Why?
Each mole of the salt has 3 moles of C2O42- ions
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thrush
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#3
Report 4 weeks ago
#3
(Original post by ChemistryWebsite)
Each mole of the salt has 3 moles of C2O42- ions
It might sound dumb but like how does that affect anything?

If you get 3 moles don't you x3?
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