need help with redox problems

[methodsboy]

hi. i need help with two redox questions:
 
1. a solution containing sulfite ions (SO32–) reacts with an acidified solution of permanganate ions (MnO4–) to produce a colourless solution contain¬ing sulfate ions and manganese(II) ions.
S(O3)^(2–)(aq) + H2O(l) ---> S(O4)^(2–)(aq) + 2H+(aq) + 2e–
MnO4–(aq) + 8H+(aq) + 5e– ----> Mn^(2+)(aq) + 4H2O(l)

^I have done the ionic equations but i don't know how to do the overall equation

2. Potassium permanganate reacts with hydrogen peroxide:
2MnO4–(aq) + 5H2O2(aq) + 6H+(aq) ---> 2Mn^(2+)(aq) + 8H2O(l) + 5O2(g)
25.0 mL of 0.02 M KMnO4 solution is reduced by 20.0 mL of H2O2 solution. What is the concentration of the hydrogen peroxide solution?
I have no idea 
Thanks in advance

[/0]

1.





You have to make the electrons on both sides cancel out. So multiply equation 1 by 5 and multiply equation 2 by 2:





And add them together:





I think that's how it goes...

2.

(ionic equation, K+ is spectator, [thanks edmund372])





Or , to 1 significant figure.

[methodsboy]

^ i got everything up to the second last line. Please explain what you did for the very last line.

[shinny]

He just cancelled things which were common on both sides. It's just like maths. If you have +10x on one side, and +16x on the other, you simplify it down to +6x. Same goes here; you're just cancelling identical substances.

[methodsboy]

i dont understand this part for question 2 :
KMnO4 isnt even in the equation  or just potassium itself???

[/0]

Sorry maybe I'm misunderstanding something here...

In the given equation there is no Potassium, so why does it say Potassium permanganate reacts?

[methodsboy]

no idea

[methodsboy]

and why did u use 5/2?

[Edmund]

K+ is a spectator ion and does not react, thus it can be ignored...

and 5/2 is the mole ratio...

[/0]

Oh... oops, it's an ionic equation isn't it?

@ methodsboy: I used 5/2 because that's the mole ratio of to

i.e.

[Edmund]

We know the moles of permanganate, so from this, we can find the moles of peroxide using the mole ratio, which is 5/2

then Working as above.


EDIT: hmmm how do you type fraction?? would have been much easier to explain 

[methodsboy]

Thanks guys, i fully get it now