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    http://www.edexcel.com/migrationdocu...e_20120307.pdf

    It is question 20 iv) of the paper in the link.

    I just doing "get" what I have to do, even after reading the mark scheme

    A brief explanation of the steps I need to take and why i need to take them would be so so helpful right now.

    thanks so much!
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    bump :/
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    -You need to calculate the number of moles of thiosulfate ions in the mean titre by multiplying the volume by the concentration (0.109x0.02735).

    -Then look at the stoichiometry to work out how many moles of chlorine were in the 10cm3 that you titrated against.

    - The question asks for the original concentration which can be found by realising that the 10cm3 used in the titration came from a diluted 250cm3 solution.

    Does this help?
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    (Original post by mattmattmatt)
    -You need to calculate the number of moles of thiosulfate ions in the mean titre by multiplying the volume by the concentration (0.109x0.02735).

    -Then look at the stoichiometry to work out how many moles of chlorine were in the 10cm3 that you titrated against.

    - The question asks for the original concentration which can be found by realising that the 10cm3 used in the titration came from a diluted 250cm3 solution.

    Does this help?
    yea, thats great thanks its just the last point I am unsure of
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    Ok. If you think about it, you started with 10cm3 of an original solution containing chlorine. THEN, you diluted this solution by making it up to 250cm3 - in doing this, the number of moles of chlorine hasn't changed but the concentration has. THEN, you took 10cm3 from the diluted 250cm3 to carry out your titration.

    SO BASICALLY, you need to look at the number of moles of thiosulfate in your titre (0.109x0.02735)
    - see that the stoichiometric ratio is 1:2, and divide this number by 2 to find the number of moles of Chlorine in the 10cm3
    - realise that there must be 25x this number of moles in the diluted 250cm3 solution form which you took the above 10cm3
    - this number of moles is the same number as in the original 10cm3. THUS, to calculate the concentration divide this number by 0.01.

    THIS --> (0.109x0.02735x0.5x25) divided by 0.01 ------> I got 3.726mold/dm3
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    What you have to know is that "made up to the mark" means taken to 250 cm3. If you then take out 10 cm3 of this and measure the Cl2 content of that, you can get the Cl2 content of the original by multiplying by 250 cm3. That would get you moles, then Molar Concentration=Moles/Volume and you have the answer.
 
 
 
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