[Rhonut]

Ok, so i have this question i'm doing for revision and i am a bit stuck. The answer on it's own is a bit useless though, i need more how to get to it and why. Anyway, here's the question:

A solution contains iron (ii) and iron (iii) sulphates, 20cm³ of this solution just reduced 19.5cm³ of 0.0200 M Potassium Manganate (vii) in acid solution. After reduction (complete), a further 20cm³ of the solution required 28.7cm³ of the potassium manganate (vii). Calculate the concentration of each sulphate in g.1^-1 present in the original solution.

I've started by getting the number of moles of manganate from the first reduction, if that's remotely correct.

[BluRay]

Sorry, it's a homework...

[Borek]

What is being oxidized during first titration?

What is being oxidized during second titration?

[Rhonut]

I'm not quite sure, that is all the information the question gives and there are no questions before that it leads from. I'm assuming the manganate is reduced, and the iron 2 is being oxidized, but how you'd get the concentration of iron 3 i don't know and why two titrations are necessary i don't know. I think the question itself is worded in a confusing way, but all i can do is try to decipher it...   I've come up with some half equations, but not sure if they're the right way to go about this:

• 5Fe²⁺  5Fe³⁺ + 5e^-
• MnO₄^- + 8H⁺ + 5e^-  Mn²⁺ + 4H₂O

I've got the number of moles of manganate used in the first titration as 0.00039 and in the second titration as 0.000574, If that's of any use..

[Borek]

iron 2 is being oxidized

That's correct. It is oxidzed in both titrations. Do you know why results of both titrations are different? There is a very important hint in the question that explains it.

[Rhonut]

It's what i'm struggling to understand. I think it may be due to the iron 3 formed during the first oxidation of iron 2, if that makes sense, i'm not sure. Does only the iron 2 get oxidised the first time and the iron 2 and iron 3 get oxidised the second time or something?

[Borek]

Does only the iron 2 get oxidised the first time and the iron 2 and iron 3 get oxidised the second time or something?

Yes!

Note that original solution contains both Fe²⁺ and Fe³⁺ and only Fe²⁺ can be oxidized. Then you pepare solution for the second titration:

After reduction (complete)

so at this moemnt it doesn't contain Fe³⁺, as it was all reduced to Fe²⁺.

You see now what have been titrated in the first titration and what have been titrated in the second?

[Rhonut]

Ah i think i can see clearly now that the rain has gone, or the Fe³⁺ rather. The difference in manganate volumes is the amount used to work out how much Fe³⁺ there was, as the Fe²⁺ oxidized by that difference in manganate is the amount of Fe³⁺. I plugged all the numbers in and my answers look relatively within reason, Thanks Borek!