The Student Room Group

Moles ratio help

http://filestore.aqa.org.uk/subjects/AQA-CHEM5-QP-JUN12.PDF

Q6di

can someone explain the mole ratio why isnt is 1:5
Reply 1
The manganate equation needs 5e- to react, and complete its reduction, add up the electrons you get from a Fe2+ oxidation and the ethanedioate ions and you get 3 in total per moles,
Then multiply be a common factor to get them to the same number of electrons, which in this case is 15, must be 3 moles of manganate and 5 moles of iron ethanedichromate
Reply 2
Original post by ImBear
The manganate equation needs 5e- to react, and complete its reduction, add up the electrons you get from a Fe2+ oxidation and the ethanedioate ions and you get 3 in total per moles,
Then multiply be a common factor to get them to the same number of electrons, which in this case is 15, must be 3 moles of manganate and 5 moles of iron ethanedichromate


I don't get it sorry, I was looking at the first two equations.

The first one has 5e- and the second equation has 1e-.

So 1:5 ratio

I don't get where the 3 comes from, even from looking at the second and third one
Reply 3
Original post by Super199
I don't get it sorry, I was looking at the first two equations.

The first one has 5e- and the second equation has 1e-.

So 1:5 ratio

I don't get where the 3 comes from, even from looking at the second and third one


Yes the first equation needs 5e- to react, the last two equations both come from the iron ethanedioate, the iron will go to Fe3+ and e-, the ethanedioate ion goes to 2CO2 and 2e-.

So for one molecule of of iron ethanedioate you get 3 electrons, 1 from the iron and 2 from the ethandioate, hence the 3:5 ratio
Reply 4
Original post by ImBear
Yes the first equation needs 5e- to react, the last two equations both come from the iron ethanedioate, the iron will go to Fe3+ and e-, the ethanedioate ion goes to 2CO2 and 2e-.

So for one molecule of of iron ethanedioate you get 3 electrons, 1 from the iron and 2 from the ethandioate, hence the 3:5 ratio


Why don't you balance the last 2 equations first?

Because I thought when you make an over reaction 'ethanedioate' you would have to balance out the electrons first

So now I'm thinking why isn't it 2:5
Reply 5
This is so weird I did this question yesterday and I balanced the equation differently and I literally spent about 20 minutes rechecking it to make sure it added up because the mark scheme said differently. It balanced fine with a different ratio. (Unless I literally don't know how to count) I can go check in my book what I did if you want.


Posted from TSR Mobile
Reply 6
I don't how I can explain this in any simpler terms, the top equation is the manganate one,
1 Mole reacts with 5 electrons.
The last two equations show what the iron ethanedioate produces in terms of electrons, iron gives one and ethanedioate gives 2. This means for ONE mole of iron ethanedioate 3 electrons are produced. The Manganate needs 5 electrons to be reduced, at least for one mole of it.
The iron ethanedioate gives 3 electrons per mole. 3 and 5 are not equal.
Make them a common factor of 15, multiply the managante equation by 3 to get 15 electrons. Hence 3 moles of manganate
Multiply the iron ethandioate equation by 5 to get 15 electrons. Now the electrons are balanced, We have 3 moles of manganate and 5 of iron ethanedioate.
If it was 2:5 then this means we used two moles of Manganate, which is 10 electrons (2x5), and 5 moles of iron ethanedioate which produces 15 electrons (3x5). This is not an equal number of electrons

Quick Reply