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Chemistry Homework Help Please

Discussion in 'Off-Topic' started by piff, Apr 5, 2004.

  1. piff

    piff IncGamers Member

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    Chemistry Homework Help Please

    I need some help for a chemistry lab report. I'll show you what numbers I have and what I need. It is about equilibrium constants.

    Net Ionic equasion:
    Fe+3(aq) + SCN- (aq) <--> FeSCN+2 (aq)

    Solution Absorbance
    1 .081
    2 .090
    3 .109
    4 .099
    5 .95
    "Equilibrium concentrations of SCN-, Fe+3, and FeSCN+2. Use absorbances to find [FeSCN+2], then find the other stoichiometrically. Show all work, using solution 1."

    Can anyone help me?

    EDIT: There's more info, but I don't think it's needed. If it is, post and I'll check back later...thanks in advance.
     
  2. Raft Boy

    Raft Boy IncGamers Member

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    I'm assuming that you listed the absorbance of the sample solution, and that somewhere you have the absorbance of a standard solution of known concentration. Absorbance is related to concentration like so:

    Sample conc/standard conc = Sample abs/standard abs

    Rearrange to solve for the sample conc, we get:

    sample conc = sample abs * standard conc/standard abs

    The sample concentration is that of the FeSCN+2. Now, you know what your concentrations of each of the ions would be in equilibrium, and you know that Keq is:

    Keq = [FeSCN+2]/([SCN-][Fe+3])

    You have 5 different samples it looks like, so calculate the average for all five, and they should be fairly close. Take the average, and presto!

    -Rafty
     
  3. piff

    piff IncGamers Member

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    Ok, all of these terms sound familiar, so nothing you are saying is something I haven't learned yet. It's not all clicking quite yet. I'm going to read it a few more times. The concentrations of solution 1 are:
    SCN-: 3.54x10^-4 M
    Fe+3: 1.52x10^-4 M

    EDIT: What's the difference between standard and sample?
     
  4. Raft Boy

    Raft Boy IncGamers Member

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    There should have been a solution that was premade that you used to run some absorbances. The premade solution should be of known concentration (whatever molarity) and you should have ran at least one absorbance on it.

    The sample is whatever you made up in lab. Looking at your numbers, I would guess five that are of different concentrations.

    I'll stick around for a bit until you figure out where to head from here.

    -Rafty
     
  5. piff

    piff IncGamers Member

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    All five are different concentrations...but we had no sample. If you need the other concentrations, let me know.
     
  6. Raft Boy

    Raft Boy IncGamers Member

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    Did you prepare both of these in lab yourself, or were these given?
     

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