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Homework Help: Determination of an unknown concentration

  1. Dec 6, 2008 #1
    1. The problem statement, all variables and given/known data
    Hello. I have a lab procedure that I am supposed to write, and since my teacher is on a two-week sick leave and the substitute does not know anything, I seriously need some assistance.

    Please note that for the following problem, a titration is NOT allowed. It must be done using Ksp and Ktrials.

    Here is the problem: There is an unknown solution that is either Lead Nitrate or Potassium Iodide.

    a) I must devise a procedure to determine which solution it is (I know how to do this part)
    b) I must devise a procedure to determine the concentration of the solution once I have determined which solution it is. There is a set list of possible concentrations that we are given.

    IF it's Potassium Iodide
    Possiblity 1: 0.05M - 0.15M
    Possibility 2: 0.20M

    IF it's Lead Nitrate
    Possiblitty 1: 0.20-0.30M
    Possibility 2: 0.05-0.015M
    Possibility 3: >0.30M

    We also given two KNOWN solutions (to use in the experiment for finding the unknown concentration sample) of both Lead Nitrate and Potassium Iodide, each having 0.01M concentration.

    2. Relevant equations
    Ksp PbI2 = 8.5 x 10^-9

    Ktrial = [Pb] ^2

    3. The attempt at a solution

    So I've been doing alot of brainstorming and I came to some conclusions that are based of qualitative observations.

    I know how to figure out part A, which is figuring out WHAT the solution is. I must simply put some of the KNOWN lead nitrate, for example, into the solution, and if I get a precpitate, I know that the solution is Potassium Iodide. And vice versa if I decide to add Potassium Iodide

    Part B is the problem. Now I realize that I can pre-calculate the Ktrial for each of the POSSIBLE concentrations of the unknown. For example, if we assume that our solution is found to be potassium Iodide, I can find out the Ktrial for both the 0.05-0.15M possibility and the 0.20M possibility. I have actually done this calculation, and I find that for the 0.05-0.15 possibility, the Q is more than the Ksp, but it is very close to it, meaning that I would see a very light precipitate, if any. With the 0.20M possibility, I have a Q substantially larger than Ksp and so I would get a dark precipitate.

    Using this QUALITATIVE property, I can thus discern a distinction and be able to state the concentration (or range of concentration) that the unknown falls within.

    My problem is that I do not think this to be a reliable method. I was wondering if anyone else has an idea as to how I can figure out part B of my procedure? Thanks alot
  2. jcsd
  3. Dec 7, 2008 #2


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    Staff: Mentor

    It is as reliable as it can get in these circumstances. Go for it.
  4. Dec 7, 2008 #3
    Hmm, well I'm glad I'm not missing something. The only problem with my method is that I must be able to tell the difference between a light, medium, and dark precipitate. I don't have a control other than my memory from an experiment two weeks ago.
  5. Dec 7, 2008 #4


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    Staff: Mentor

    No, you have to be able to tell the difference between YES, THERE IS A PRECIPIATATE and NO, THERE IS NO PRECIPITATE.
  6. Dec 7, 2008 #5
    Oh but that's the problem. I don't want to ask you to do the calculations, but if you do, you will find that there is no dilution that gives a ppt for one possibility and does not for the other possibilities. I tried 20 mL final volumes, with each solution starting at 10mL, then sequentially dropping down to 2mL (every mixture is raised to 20mL using water). None of them give a Q that is smaller than Ksp; they are either very close to it, a bit above it, or extremely above it.
  7. Dec 7, 2008 #6


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    Staff: Mentor

    Then all you will be able to say is what is the upper limit to Kso.
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