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    hey guys

    S2O82-(aq) + 2I-(aq) ® 2SO42-(aq) + I2(aq)
    For my coursework I want to work out the order of this reaction using Colorimetry. In order to do this I have to make a series of reaction mixtures where I vary the concentration of iodide while keeping the concentration of the S2O82- constant, then measure the absorbance of these reactions. Then I have to vary the concentration of S2O82- while keeping the concentration of the iodide constant then measure the absorbance of these as well. I am confused about several things:

    - I thought that the reactant that I keep constant would have to be in excess to ensure that it completely oxidizes the reactant that I vary the concentration of. But I was told to have in excess the reactant that I change the concentration of. I don’t understand why this is so. Can someone explain this to me please?

    -Also, I was told to take the mole ration into account. So when I’m varying the concentration of iodide, as well as making sure it is in excess to the S2O82- , I need to ensure that iodide is present in double the moles of the S2O82- because it has a 2:1 ratio. This works fine. But when it comes to varying the concentration of S2O82- , again I need to ensure it is in excess but because of the 2:1 ration, I can’t have S2O82- in excess to the iodide… how do you ensure that the S2O82- is in excess but at the same time have half the moles of the iodide?… I am so confused. Can someone please explain how this works?

    Thanks in advance
 
 
 
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