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Posted by on Saturday, June 28, 2008 at 12:34am.

The standard solution of FeSCN2+ (prepared by combining 9.00 mL of 0.200 M Fe(NO3)3 w/1.00 mL of 0.0020 M KSCN) has an absorbance of 0.550. If a trial's absorbance is measured to be 0.350 and its initial concentration of SCN– was 0.0010 M, the equilibrium concentration of SCN– will be what?

  • Chemistry - , Saturday, June 28, 2008 at 1:01am

    Fe(NO3)3 + SCN^- ==> FeSCN^+2

    mols Fe(NO3)3 = M x L
    mols SCN^- = M x L.
    Obviously, SCN^- is the limiting reagent. (SCN^-) = mols/L total solution OR, if my arithmetic is ok, (FeSCN^+2) = 0.0002. Check me out on that.
    Then A = abc.
    You know A = 0.550 for the standard. You know c = 0.0002 M. You can substitute any number for b (the cell length) as long as you use the same cell length for all calculations (I assume you used the same cell length for all measurements). That leaves a as the only unknown, which in this case is the molar absorptivity constant (which is usually written as epsilon but I can't write an epsilon with on this board). Then for the trial's value,
    A = abc. You know A. You know a (from the previous calculation), you know b and you can calculate c, the equilibrium concn of the FeSCN^+2. Then set up an ICE chart to determine how much of the SCN remains. It will be 0.001 - amount FeSCN^+2 formed. Check my thinking.

  • Chemistry - , Saturday, June 28, 2008 at 8:29pm

    Hey I know this isn't an answer, but i have the same problem and i'm having trouble with it. If you figured it out, could you help me with the answer?

  • Chemistry - , Saturday, June 28, 2008 at 11:29pm

    Jenifer--I told Tom how to work the problem. If yours is like it, just follow the same instructions. However, please repost it. Just above this are a couple others by UN. Look at those, also.

  • Chemistry - , Sunday, June 29, 2008 at 4:02pm

    thanks DrBob

    got it right!

  • Chemistry - , Tuesday, October 7, 2008 at 4:51pm

    For some reason, this isn't working out for me. Can you further help me? I am very confused.

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