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# Redox titrations

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How do i do question 7a?
2. Okay, so you know the permanganate reduction half-equation (the first equation).

You also know the full equation.

Since two half-equations make up the full equation, you can deduce what the hydrogen peroxide oxidation half-equation is by subtracting the permanganate half-equation species from the overall equation, leaving you with the hydrogen peroxide half-equation
3. The way I did this question was to look at the first equation on the page.

You can see that it is MnO4- + 8H+ +5 e- --> Mn2+ + 4H2O.

In the second equation, it is 2MnO4-... Etc etc.

You have to times the first equation by two to get 2MnO4- for the second equation, so do that now.

2MnO4- + 16H+ + 10e- --> 2Mn2+ +8H2O.

If you think of it as two equations, you can take away the new equation above from the second equation, so you get :

2MnO4- + 6H+ + 5H2O2 - 2MnO4- - 16H+ - 10e- --> 2Mn2+ + 8H2O + 5O2 - 2Mn2+ - 8H2O

Cancel anything out if you can and you get left with :

5H2O2 -10H+ - 10e- --> 5O2

Move all the 'negative' signs to the other side (in those case, it's the H+ and the e-) and change them to 'positives'

5H2O2 ---> 5O2 + 10H+ + 10e-

Simplify (everything goes into 5 so divide by 5)

H2O2 --> O2 + 2H+ + 2e-

Did you understand the process I went through?

This was posted from The Student Room's iPhone/iPad App
4. (Original post by TiTo20)
The way I did this question was to look at the first equation on the page.

You can see that it is MnO4- + 8H+ +5 e- --> Mn2+ + 4H2O.

In the second equation, it is 2MnO4-... Etc etc.

You have to times the first equation by two to get 2MnO4- for the second equation, so do that now.

2MnO4- + 16H+ + 10e- --> 2Mn2+ +8H2O.

If you think of it as two equations, you can take away the new equation above from the second equation, so you get :

2MnO4- + 6H+ + 5H2O2 - 2MnO4- - 16H+ - 10e- --> 2Mn2+ + 8H2O + 5O2 - 2Mn2+ - 8H2O

Cancel anything out if you can and you get left with :

5H2O2 -10H+ - 10e- --> 5O2

Move all the 'negative' signs to the other side (in those case, it's the H+ and the e-) and change them to 'positives'

5H2O2 ---> 5O2 + 10H+ + 10e-

Simplify (everything goes into 5 so divide by 5)

H2O2 --> O2 + 2H+ + 2e-

Did you understand the process I went through?

This was posted from The Student Room's iPhone/iPad App
I dont understand how you got this part

OH WAIT you mean cancel eaach side of the equation by itself not opposite sides?!!
5. (Original post by thegodofgod)
Okay, so you know the permanganate reduction half-equation (the first equation).

You also know the full equation.

Since two half-equations make up the full equation, you can deduce what the hydrogen peroxide oxidation half-equation is by subtracting the permanganate half-equation species from the overall equation, leaving you with the hydrogen peroxide half-equation

(Original post by TiTo20)
The way I did this question was to look at the first equation on the page.

You can see that it is MnO4- + 8H+ +5 e- --> Mn2+ + 4H2O.

In the second equation, it is 2MnO4-... Etc etc.

You have to times the first equation by two to get 2MnO4- for the second equation, so do that now.

2MnO4- + 16H+ + 10e- --> 2Mn2+ +8H2O.

If you think of it as two equations, you can take away the new equation above from the second equation, so you get :

2MnO4- + 6H+ + 5H2O2 - 2MnO4- - 16H+ - 10e- --> 2Mn2+ + 8H2O + 5O2 - 2Mn2+ - 8H2O

Cancel anything out if you can and you get left with :

5H2O2 -10H+ - 10e- --> 5O2

Move all the 'negative' signs to the other side (in those case, it's the H+ and the e-) and change them to 'positives'

5H2O2 ---> 5O2 + 10H+ + 10e-

Simplify (everything goes into 5 so divide by 5)

H2O2 --> O2 + 2H+ + 2e-

Did you understand the process I went through?

This was posted from The Student Room's iPhone/iPad App
Thank you very much for your help
For b i done:
4.69x10-4/2 =2.345x10-4 moles
1.1725x10-3/0.025 =0.0469 mol dm-3
but for gdm im assuming you: mass x volume?
so mass is nx mr = 1.1725x10-3x34 = 39.865 and volume is 5.499 x10-5

39.865 x 5.499x10-5

so i got 2.19x10-3 gdm-3

ive really messed it up but i dont know how or where such a fail.
6. (Original post by asaaal)
Thank you very much for your help
For b i done:
4.69x10-4/2 =2.345x10-4 moles
1.1725x10-3/0.025 =0.0469 mol dm-3
but for gdm im assuming you: mass x volume?
so mass is nx mr = 1.1725x10-3x34 = 39.865 and volume is 5.499 x10-5

39.865 x 5.499x10-5

so i got 2.19x10-3 gdm-3

ive really messed it up but i dont know how or where such a fail.
The first two bits are right, but you've forgotten the volume that has that about of moles in it. Remember, you USED 25cm^3 when there was 250cm^3 to START off with, so the number of moles of H2O2 is times by ten.

Think you can take it from here?

This was posted from The Student Room's iPhone/iPad App
7. (Original post by TiTo20)
The first two bits are right, but you've forgotten the volume that has that about of moles in it. Remember, you USED 25cm^3 when there was 250cm^3 to START off with, so the number of moles of H2O2 is times by ten.

Think you can take it from here?

This was posted from The Student Room's iPhone/iPad App

Wait so how do i get gdm?
8. (Original post by asaaal)
Wait so how do i get gdm?

This was posted from The Student Room's iPhone/iPad App

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