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roff
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Aim: The purpose it to balance the equation for the reaction between sodium thiosulphate and iodine, however, we don't know the values of 'a' and 'b'


'a' Na2S203 + 'b'I2 -> Products

I have to determine the radio of ao to b by taking a known amount of Iodine and titrating it with standard sodium thiosulphate

What I know so far

Pipette sol: iodine solution at 0.05 mol dm^3 concentration
Burette sol: sodium thiosulphate .. concentration unknown
Indicator: starch solution

Volume used (titre)/cm^3 91.4
Mean titre/cm^3 18.28

Use the results to determine the stoichiometric coefficients 'a' and 'b' in the equation given above.




2nd question: all the iodine forms sodium iodide, NaI, there is one othe rproduct - work out its formula




All the Chem whizzes, help would be great!

cheers
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C4>O7
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You will need the thiosulphate concentration- otherwise you cannot calculate the moles of Na2S203. Once you have found the molar ratios you can you can find a and b.
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roff
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How do I find that out the tiosulphate concen?
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C4>O7
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If you know a and b you can find the concentration; alternatively if you have the concentration you can find a and b!
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roff
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(Original post by C4>O7)
If you know a and b you can find the concentration; alternatively if you have the concentration you can find a and b!
Could you go into a bit more detail there pls? :confused:
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roff
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anyone?
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hitchhiker_13
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You have to tell us the concentration of the standard sodium thiosulphate solution before we can help you.
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roff
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But the concentration of thiosulphate isn't mentioned at all, only the concen. for iodine.. so it is 100% impossible to go any further without knowing the thiosulphate concen?
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ChemistBoy
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How can you use "standard thiosulfate solution" and not know the concentration?
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roff
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(Original post by ChemistBoy)
How can you use "standard thiosulfate solution" and not know the concentration?
Well it doesn't mention it and the teacher didn't say what it was.. If you do know please just say so, and how I would go about solving 'a' and 'b'
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hitchhiker_13
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(Original post by ramroff)
Well it doesn't mention it and the teacher didn't say what it was.. If you do know please just say so, and how I would go about solving 'a' and 'b'
Ok, forget the concentration.
What volume of iodine did you add?
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roff
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(Original post by hitchhiker_13)
Ok, forget the concentration.
What volume of iodine did you add?
10cm^3
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ChemistBoy
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I don't really like doing titrations. You need to consider the reaction.

aNa2S2O3 + bI2 ----> cNaI + another product.

lets look at half reactions Na+ is just a spectator here so we can ignore it.

I2 + 2e- -----> 2I-

so

2S2O3(2-) ------> X? + 2e- in order to balance electrons. I'll give you a clue that X is one mole equivalent in that equation (i.e. a single species is formed from two ions).

Once you've got the equation, then you can work out thiosulfate concentration from your titre.

PM me again if you need more help.
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hitchhiker_13
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(Original post by ChemistBoy)
I don't really like doing titrations. You need to consider the reaction.

aNa2S2O3 + bI2 ----> cNaI + another product.

lets look at half reactions Na+ is just a spectator here so we can ignore it.

I2 + 2e- -----> 2I-

so

2S2O3(2-) ------> X? + 2e- in order to balance electrons. I'll give you a clue that X is one mole equivalent in that equation (i.e. a single species is formed from two ions).

Once you've got the equation, then you can work out thiosulfate concentration from your titre.

PM me again if you need more help.

But haven't you used the mole ratio for that?
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ChemistBoy
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You can't do both, you need the stoichiometry or the concentration. If you are titrating an unknown concentration solution and you don't know the reaction stoichiometry then how do you know what is going on? In this example you can use chemical intuition to work out the stoichiometry. The titre is useless as thiosulfate concentration could be anything so you can't workout how many moles of thiosulfate have reacted and hence deduce the stoichiometry. I'm convinced that the 'standard sodium thiosulfate solution' will have a stated concentration - if it doesn't the experiment is pointless.
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hitchhiker_13
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Exactly - you must have forgotten to take it down or something ramroff.
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