[Rayan]

the aim is to determine the normality of a potassium permanganate solution.
we  mixed H₂SO₄ with KI and KMnO₄, the following rxn takes place:

2knO₄ + 10 KI + 8 H₂SO₄ ---> 5I₂ + ...

then we titrated the I₂ by a Na₂S₂O₃ solution, the rxn is:

I₂ + 2S₂O₃^2- ---> S₄O₆^2- + 2I^-

so from the second equation, we calculate the normality of the thiosulfate solution, and then we have n KMnO₄ = n Na₂S₂O₃
(this is written in the manual)

but what is the number of moles of I₂?
we should calculate it from the first equation, but is it just simply nI₂/5 = nacid/8 ?
and if so, then n = CV. what is the volume? the total volume of solution?

[hmx9123]

Once you have the number of moles of KMnO₄, it necessarily follows that you have I₂ in a 5:2 ratio with KMnO₄.

When you say n = CV, what are you meaning by CV?  Variables?  Concentration times volume?  The number of moles is equal to the concentration times the volume of the solution you are using.  If you're trying to determine the number of moles of permanganate, I don't know where the manual got that the number of moles of permanganate is equal to the number of moles of thiosulfate from what you have written.  I would assume that one mole of permanganate produced 5 moles iodine and each mole of iodine titrated two moles of thiosulfate, so one mole of permanganate actually corresponds to ten moles of thiosulfate.  Therefore, if you have the number of moles of thiosulfate that you used, (not molarity, but moles), divide it by 10 and you'll have the number of moles of permanganate.  Since you know how much permanganate solution you used to begin with (I'm sure you recorded this volume, or this whole exercise is pointless), you can calculate concentration.

[Rayan]

okay but what is the number of moles of I₂? at equivalence we have the number of moles of I₂ is equal to that of thiosulfate so i can calculate the number of moles of thiosulfate and then permanganate. and TWO mole of permanganate produce 5 moles of I₂ and each one mole of I₂ titrates 2 moles of thiosulfate, then five moles of permanganate correspond to one mole of thiosulfate and not ten.
now going back to the number of moles of I₂, where should i get it from?

[Rayan]

what is the relation between normality N and the molarity? i remember it is M = N/x where x is the number of electron exchanged. but i'm not sure..
and in this case, and if it true the relation i wrote, for KMnO₄ , M = N/5 and for S₂O₃^2- M= N/2 ? is it correct like this?

[xiankai]

i take it u meant one mol of permaganate corresponded with five mol of thiosulfate? because thats the correct answer.

if u want to find the total number of moles of I2, u should refer to your titration results as to how much thiosulfate was used, then divide it by 2.

[hmx9123]

Sorry, my math error.  If 2 mols permanganate give 5 mols iodine, which titrate 10 mols thiosulfate, 1 mol of permangante will titrate 5 mols thiosulfate.  

The number of mols of iodine is equal to half the number of mols of thiosulfate, as xiankai states, because they are in a 1:2 ratio of iodine to thiosulfate.

Normality is the number of equivalents per liter of solution.  If we're making permanganate go to manganese(II), we're talking about 5 electron transfer, and so one mol of permanganate consumes 5 equivalents of electrons to give the manganese(II) ion.  This means that 1 mol of permanganate is equal to 5 equivalents of permanganate, or 1M KMnO₄ is equal to 5N KMnO₄, so you are correct in your ascertation about normality and molarity relations in the case of permanganate.  Take a look at the oxidation state changes in sulfur in thiosulfate and see if you can come up with rationale either supporting or refuting your statement that 1M thiosulfate is equivalent to 2N thiosulfate.  It sounds like you have a decent handle on this information.

[Rayan]

no i'm not correct!
u said that 1 M KMnO₄ is equal to 5N, i said it it equal to N/5, that is, i said that 1N = 5M. i must have remembered it wrong.. and the correct formula must be N = M/x.
in case of thiosulfate, S₂O₃ ^2- is transformed into S₄O₆ ^2- , thus we need two electrons, and then we get M = 2N .  is this correct?

now let's go back to the titration. usually when we want to calculate the normality of a solution indirectly, we dont just mixe the solution and then titrate only once!
i mean that, from the titration, i get the V of thiosulfate needed to titrate I₂. the normality of thiosulfate is unknown. so at equivalence, the reactants react according to the stochiometric ratios, and nI₂ = n thiosulfate/2 thus 2 N.V (iodine) = N.V (thiosulfate).
so "given" the number of moles of iodine, i calculate the normality of thiosulfate and then according to their relation written in the manual (and which i think is wrong) NV (permanganate) = NV (thiosulfate) we calculate the normality of permanganate.
and so we come back to the same problem, how to calculate the number of moles of iodine!
because when usually "I" do such an experiment, we dont just mixe at the first, we do a titration too, and fromt he titration we get the number of moles of iodine and then do a second the titration with thiosulfate and calculate its normality.
2kMnO₄ + 10 KI + 8 H₂SO₄ ---> 5I₂ + ...

here we have the volume of KI added. and they said it's 5-10 % (i dont know what that is), and the volume of the acid with its normality and the volume of KMnO₄. but the acid, isnt just added to make an acidic medium? how can i know the limiting reagant to calculate the number of moles of iodine produced!!

i'm so sorry about the long post.. but there's one more thing, about the relation the wrote:
NV (permanganate) =  NV (thiosulfate)

i would have said that, nI₂ from the above equation is equal to 5/2 nKMnO₄. and nI₂ using the titration equation is n thiosulfate/2, so we have:
n / 2 thiosulfate = 5/2 n permang.
so n permanganate = n thio /5
so, using the normalities, we get:

5 N. V (permang.) = 2 N.V (thio)

and not NV (permang.) = NV (thio) as they said, isnt correct?

thank you very much.

[hmx9123]

u said that 1 M KMnO4 is equal to 5N, i said it it equal to N/5, that is, i said that 1N = 5M. i must have remembered it wrong.. and the correct formula must be N = M/x.

I misinterpreted what you wrote.  I thought you meant: molarity equals normailty divided by five.  That is correct.  I don't understand your formula, so if it works for you, great.

in case of thiosulfate, S2O3 2- is transformed into S4O6 2- , thus we need two electrons, and then we get M = 2N .  is this correct?

1 M = 2 N.  Yes.

I don't understand how you're supposed to titrate something without a standardized solution.  If you don't know the concentration of the thiosulfate, and you don't know the concentration of the permanganate, how are you supposed to figure out the mols of iodine?  It's all nebulous.

KI is 5-10%?  Is this the concentration of the solution?  How much you're supposed to add?  Do you have a standardized solution of KI?  I'm confused.

What you have written down for the 5 NV (perman) = 2 NV (thiosulf) is correct for molarity.  I don't think it's correct for normality.  I think because normality is based on equivalences, you wind up with a 1:1 ratio.  Unfortunately, I am way too tired to think at the moment.  I hate normality.  I have never heard a convincing argument for its use anyway.  Sorry not to be of so much help there, but perhaps tomorrow when I've slept a little more, I'll be able to figure it out.

[Borek]

here we have the volume of KI added. and they said it's 5-10 % (i dont know what that is), and the volume of the acid with its normality and the volume of KMnO₄. but the acid, isnt just added to make an acidic medium? how can i know the limiting reagant to calculate the number of moles of iodine produced!!

Limiting reagent is permanganate.

As you have stated in your first post yor aim is to find out concentration of permanganate solution.

To do so you should take known volume of permanganate, add excess of iodide and sulfuric acid. Amount of iodine evolved is directly proportional to amount of permanganate. Next step is to titrate iodine with thiosulfate solution of KNOWN concentration.

Amount of iodine is a link between amount of permangante and thiosulfate.

As hmx pointed out - if you don't know the concentration of thiosulfate solution you can't do anything.