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AS Chemistry c/w titrations URGENT!!

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    I need to answer some questions as part of a practical we did today, the calculation questions are fine but there are some that are more about technique and I need you to help me with them.

    The titration was titrating sodium thiosulphate with iodine. To get the iodine we mixed our solution of sodium thiosulphate (aq) with potassium iodide and hydrochloric acid.

    The equations are -

    2Na2S2O3 + I2 => 2NaI + Na2S4O6

    or ionically,

    2S2O3 (2-) + I2 => 2I (-) + S4O6 (2-)

    for the titration and for the mixing of the 3 chemicals to get the iodine the equation is ionically-

    IO3 (-) + 5I (-) + 6H (+) => 3I2 + 6H2O.

    The questions are...

    + Weighing, the use of pipettes and burettes and the preparation of the solution of KIO3 (V) are likely to be sources of error in the experiment - discuss two sources of errors and which of these 4 is to produce most variation and explain.

    There is obviously one error from each thing mentioned.

    + For one of the errors suggest whether the error is likely to cause an increase or a decrease in the percentage purity.

    + What is the reason for using so much 1M sulphuric acid?

    If anyone can help I would really appreciate it.

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    Well I handed it in today, and it went ok. Cheers for the plentiful responses.
Updated: March 1, 2005
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