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How many milligrams of Vitamin C were in the original tablet?

  1. Feb 19, 2012 #1
    1. The problem statement, all variables and given/known data

    You ground your vitamin C tablet and dissolved it in 250. mL of water in a volumetric flask. You used a 25.0 mL aliquot of this solution for your titration. If it takes 9.40 milliliters of your 0.0151M titrant, how many milligrams of ascorbic acid (MW 176.12) were in the original tablet?

    2. Relevant equations

    M1V1=M2V2

    3. The attempt at a solution

    0.015 M x 0.0940 L = M 2 V2

    I'm not sure I'm using the right equation.
     
  2. jcsd
  3. Feb 19, 2012 #2

    Borek

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    Staff: Mentor

    If you are not told what the titrant is, question doesn't make sense. And M1V1=M2V2 is not a "relevant equation"when it comes to titrations, it is a specific case which may, or may not work.

    Compare

    http://www.titrations.info/titration-calculation

    for a very basic example of the titration where this equation doesn't work.
     
  4. Feb 19, 2012 #3
    The titrant of this experiment is triiodide, but it didn't have that info in the question.
     
  5. Feb 20, 2012 #4

    Borek

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    Staff: Mentor

    What is the reaction? Or more precisely - what is stoichiometry of the reaction? In what molar ratio do ascorbic acid react with iodine (or triiodide, it is the same in this context)?
     
  6. Feb 22, 2012 #5
    Reaction: C6H8O6 + I3- +H2O ↔ C6H6O6 +3I- +2H3O+
    Stoichiometry is one to one.

    So 9.40 mL of 0.015 M titrant contains 1.41x10^-4 mol I3-.
    Since it's a 1:1, 25mL aliquot contains 1.41x10^-4 moles vitamin C.
    There are 10 25mL in 250 mL so 10 x 1.41x10^-4 moles = 1.41 x 10^-3 moles
    1.41x10^-3 x 176.12 = 0.2483292g = 248.3 mg.

    Are my sig figs correct?
     
  7. Feb 22, 2012 #6

    Borek

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    Staff: Mentor

    Don't round down intermediate results - that is, round them down for reporting, but use as many digits as you have for further calculations.

    Your final result is slightly off (I suppose that's because of this rounding down). Both volume and concentration of the titrant are given with three significant digits, so your final answer should have three digits as well.
     
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