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Homework Help: Understanding the chemistry behind: Why O2 and not I2?

  1. Mar 11, 2017 #1
    • Please post this type of questions in HW section using the template.
    We were doing O2 estimation by Winkler's method

    The procedure and reactions involved in this experiment are:
    1. Carefully fill a 300-mL glass stoppered bottle brim-full with sample water.
    2. Immediately add 2mL of manganese sulfate to the collection bottle.
    3. Add 2 mL of alkali-iodide reagent (NaOH + KI) in the same manner.
    4. Stopper the bottle with care to be sure no air is introduced. Mix the sample by inverting several times.
    5. Add 2 mL of concentrated sulfuric acid.Then mix well.
    6. In a glass flask, titrate 100 mL of the sample with sodium thiosulfate to a pale straw color. Titrate by slowly dropping titrant solution from a calibrated pipette into the flask and continually stirring or swirling the sample water.
    7. Add 2 mL of starch solution so a blue color forms.
    8. Continue slowly titrating until the sample turns clear.
      Now the formula to calculate the amount of O2 is


      Well don't we use the formula V1 N1 = V2 N2 only when two substances, compound-1 and compound-2 are reacting?
      If we look at the reactions 1 Na2S2O3 is not reacting with 1O2 but 1/2 molecule of I2. Well then again may be had the number of I2 produced by one O2 was the same as the number of I2 reacting with Na2S2O3, we could still use O2 in place of I2. But here it is not so.
      This doesn't make sense at all.

      Summing up: Why/How do we use this formula with O2 and not I2 when it is not reacting with Na2S2O3?
    Last edited: Mar 11, 2017
  2. jcsd
  3. Mar 11, 2017 #2


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    Staff: Mentor

    A lot depends on how you define the normality of thiosulfate.

    In general you should derive the formula following the stoichiometry. Can you write the overall reaction? Intermediates should cancel out and in the end you should get the correct stoichiometric ratio between oxygen and thiosulfate.
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