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Homework Help: Chemistry Equilibrium Lab Question

  1. Feb 13, 2010 #1
    1. The problem statement, all variables and given/known data

    Hi! Okay so I have a question about the calculations for an equilibrium lab I completed. I have the equation

    Fe3+ + SCN1- makes FeSCN2+

    I know that Fe3+ equals 2.25x10 to the -5 power M and SCN1- equals 0.50 M.

    . The attempt at a solution

    Would I simply add the 2 together to get FeSCN2+? That seems too easy but it's all I can think of...thanks!
  2. jcsd
  3. Feb 20, 2010 #2
    First of all assume you have 1 litre of sol'n
    so you have 2.25x10^-5 mols of Fe3+ and 0.50 mols of SCN-
    The Fe3+ is the limitting reagent so you can only get 2.25x10^-5 mols of FeSCN2+.
    (Imagine you are making Ham and cheese sandwhiches. You have 20 pieces of cheese and 5 pieces of ham. You can only make 5 sandwhiches. Only instead of cheese it is SCN- and instead of Ham it is Fe3+)
    So you have 2.25x10^-5 mols of FeSCN2+ and we assumed we had 1 litre of Fe3+ sol'n and 1 litre of SCN- sol'n. So when we put them together we had 2 litres of sol'n
    so the concentration of FeSCN2+ = 2.25x10^-5 mols/2 litres
  4. Feb 21, 2010 #3


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    Staff: Mentor

    That would mean concentration of Fe3+ is zero and stability constant is infinite. So it is not a correct approach.

    xoilovemeryl: data you have listed so far is not enough to calculate concentration, it can be only approximated using stoichiometry (as Iodine did), but the value obtained is not the real one. I suppose you had more data to use in your calculations.

  5. Feb 21, 2010 #4
    This was a lab, so just as Borek said, you cannot just assume 1L of each. What if he used 1L of Iron and 10mL of SCN?
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