Super199
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http://filestore.aqa.org.uk/subjects...5-QP-JUN14.PDF

8b)

Can someone explain in really simple terms. I don't seem to understand
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charco
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(Original post by Super199)
http://filestore.aqa.org.uk/subjects...5-QP-JUN14.PDF

8b)

Can someone explain in really simple terms. I don't seem to understand
OK, your question (b) depends on the method used to determine the mass in part (a)

A student carried out an experiment to find the mass of FeSO4.7H2O in an impure sample, X.The student recorded the mass of X. This sample was dissolved in water and made up to 250 cm3 of solution.
The student found that, after an excess of acid had been added, 25.0 cm3 of this solution reacted with 21.3 cm3 of a 0.0150 mol dm–3 solution of K2Cr2O7


This determination is a redox titration. The iron(II) is oxidised to Fe(III) and the dichromate is reduced from (VI) to (III)

Hence the ratio of reactants is 1 iron to 3 chromium, and as there are two chromium atoms in dichromate, the ratio is 6 iron ions to 1 dichromate ion:

6Fe2+ + Cr2O72- + 14H+ --> 6Fe3+ +2Cr3+ +7H2O

So the titration gives you the moles of dichromate = 0.0213 x 0.015 = 3.195 x 10-4 and from this you can work out the moles of iron = 6 x 3.195 x 10-4= 1.917 x 10-3
... and from this you calculate the mass of the iron sulfate.

The only way for the final value to be too high is if there is too high a value for the dichromate titration.

You are told that there is an impurity that has caused the dichromate to have too high a mole value from the titration, hence the impurity MUST have also reacted with the dichromate ions.

This would give a higher value for the titre, more moles of dichromate, more moles of iron calculated and a higher mass determined.
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Super199
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(Original post by charco)
OK, your question (b) depends on the method used to determine the mass in part (a)



This determination is a redox titration. The iron(II) is oxidised to Fe(III) and the dichromate is reduced from (VI) to (III)

Hence the ratio of reactants is 1 iron to 3 chromium, and as there are two chromium atoms in dichromate, the ratio is 6 iron ions to 1 dichromate ion:

6Fe2+ + Cr2O72- + 14H+ --> 6Fe3+ +2Cr3+ +7H2O

So the titration gives you the moles of dichromate = 0.0213 x 0.015 = 3.195 x 10-4 and from this you can work out the moles of iron = 6 x 3.195 x 10-4= 1.917 x 10-3
... and from this you calculate the mass of the iron sulfate.

The only way for the final value to be too high is if there is too high a value for the dichromate titration.

You are told that there is an impurity that has caused the dichromate to have too high a mole value from the titration, hence the impurity MUST have also reacted with the dichromate ions.

This would give a higher value for the titre, more moles of dichromate, more moles of iron calculated and a higher mass determined.
Ok so umm the impurity has reacted with dichromate causing more of it to form? If so how?

I get the second bit in bold I think. Just how the impurity/ or what it is reacted with the dichromate?

Thanks
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TheTennisOne
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(Original post by Super199)
Ok so umm the impurity has reacted with dichromate causing more of it to form? If so how?

I get the second bit in bold I think. Just how the impurity/ or what it is reacted with the dichromate?

Thanks
The impurity is a reducing agent like Fe 2+ but must react with more Chromate than the first Fe
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Super199
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(Original post by TheTennisOne)
The impurity is a reducing agent like Fe 2+ but must react with more Chromate than the first Fe
How did you know that sorry?
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TheTennisOne
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(Original post by Super199)
How did you know that sorry?
Because if it reacted with the same amount of Dichromate or less; the predicted mass as per the ratio above would be the same as or less than the weighed mass, so it must reduce more than the first Fe edit: and must reducing agent as it generates the reduced chromate
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Super199
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(Original post by TheTennisOne)
Because if it reacted with the same amount of Dichromate or less; the predicted mass as per the ratio above would be the same as or less than the weighed mass, so it must reduce more than the first Fe edit: and must reducing agent as it generates the reduced chromate
Safe x
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the don
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Jesus Christ, Did my A-Levels in 2013. Chem 5 is some really painful stuff. I don't remember much but people, it gets much easier at Uni! Even thinking about it gives me the shivers!
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