AQA A Level Chemistry REDOX titration q

Suggest one property of an impurity that would cause the calculated mass of FeSO4.7H2O in X to be greater than the actual mass of X?

Would anyone be able to explain the answer to this q please?
(Impurity is a) reducing agent / reacts with dichromate / impurity is a
version of FeSO4 with fewer than 7 waters (not fully hydrated)
Such that for a given mass, the impurity would react with more dichromate
than a similar mass of FeSO4.7H2O

Reply 1

Philipboi

Original post by anon25x

So if the impurity is Iron(II) Sulphate that's not fully hydrated, say for example, it's completely dehydrated, then for the same amount of moles (obtained from the redox titration) your actual mass (ie factoring in the impurity) will be less than the mass you calculate, from doing the moles times by the Mr of hydrated Iron(II)Sulphate.

For the reducing agent point, the idea is that if the impurity is not able to reduce/react with dichromate then it won't affect the titration, as you're simply measuring the moles of dichromate that react until it's in excess, then using that to get a value for how many moles of Iron(II) sulphate must have been present. This is a similar idea to the fact that in acid/base titrations, you can put as much water in the conical flask as you like, as it doesn't effect the actual amount of acid/base in the flask, so the amount of reaction is the same.
Hence, if you've got an impurity that doesn't get involved in the reaction (ie doesn't reduce dichromate) then the moles of Iron(II) Sulphate you think you have, will be larger than the amount actually in the sample X.

Hope this helped!