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Homework Help: Redox titration help

  1. Mar 20, 2008 #1
    [SOLVED] Redox titration help

    In class we are doing a redox titration with a diluted bleach solution. We made the aliquot with 5 ml of 10% KI solution and 5ml H2SO4 with 25 ml of the diluted bleach solution. And the burrette solution was sodium thiosulfate @ 0.1 mol L-1. my results were as follows... 18.2, 18.2 and 18.3

    OCL- + 2H+ + 2S2O3-2 = Cl- + S4O6-2 +H2O

    I know that the average volume used is 18.23. The question asks to calculate the concentration of hypochlorite ions in diluted bleach solution and to calculate the concentration of NaOCl in grams per liter. please could someone help from this point as I am completely stuck....
  2. jcsd
  3. Mar 20, 2008 #2
    you equation says that 2 mol of thiosulphate reacts with 1 mol hypochlorite.

    how many mols of thiosulphate was needed from burette? then, how many mols of hypochlorite was present in the solution? this amount of hypochlorite is found in what volume?

    use proportion.
  4. Mar 20, 2008 #3
    How do you calculate moles again?
  5. Mar 20, 2008 #4
    your thiosulphate concentration is 0.1M. this implies that in 1000 ml you will have 0.1 mol of thiosulphate. how many mol of thiosulphate you will have in the average volume from the burette? use proportion.

    now this amount of thiosulphate will react with hypochlorite. but according to the stoichiometric equation, 2 mol of thiosulphate reacts with 1 mol of hypochlorite.

    can you find out how many mols of hypochlorite reacted with the number of mol of thiosulphate you just calculated above?
  6. Mar 21, 2008 #5
    I got that the number of moles of thiosulphate is 0.001823 mol. And because the equation is 2 mol thiosuphate for 1 mol hypochlorite so I divide by 2 to get that the number of moles of hypochlorite is 0.0009115. Is that right?
  7. Mar 21, 2008 #6

    how you find the concentration of hypochlorite now?!
  8. Mar 21, 2008 #7
    c=n\v so concentration of hypochlorite will be 0.0009115 divided by 0.0250 which is 0.03646 mols per liter. Is that correct? what do I do from here to find the concentration in grams per liter?
  9. Mar 21, 2008 #8
    i think it's much easier to use proportion than formulae with these calculations. it's up to you though.

    so, 25 ml of hypochlorite contains 0.0009115
    1000 ml would contain _________ mol

    what is the mass of 1 mol NaOCl? hint: Mr

    and always give your answers in scientific notation. e.g 9.115 * 10^-4
  10. Mar 22, 2008 #9
    So does that mean that the mass of NaOCl in grams per liter would be the concentration times the relative molar mass?
  11. Mar 22, 2008 #10
    yeah....... if the solution contains x mol per liter, the x mol also corresponds to a certain mass. then you have x*r.m.m. g/L
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