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Insoluble precipitates

  1. Jun 30, 2016 #1

    TT0

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    1. The problem statement, all variables and given/known data
    A student was given a sample of a colourless solution containing three cations and was asked to identify the cations. The student carried out a series of reactions as shown in the flow chart below:

    Screen Shot 2016-06-30 at 3.55.20 PM.png

    From the information above, suggest three cations which could be in the sample and would form the precipitates A, B and C. Give the formula for each precipitate.

    2. Relevant equations


    3. The attempt at a solution

    I think that precipitate A is AgCl because AgCl is insoluble. This means one of the cations is Ag+. However, I am stuck and don't really know how to move on. In the second step, Na2SO4 solution is added to the AgCl. I am not sure what precipitate will be formed. I am guessing that it has something to do with the other cations but I don't really know. I am pretty sure I have misinterpreted the question so could some please explain where I went wrong?

    Cheers!
     
  2. jcsd
  3. Jun 30, 2016 #2

    Borek

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    Staff: Mentor

    You are right about AgCl.

    What insoluble sulfates do you know? Hydroxides?

    It is all about solubility rules.
     
  4. Jun 30, 2016 #3

    TT0

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    Calcium sulfate, barium sulfate, lead sulfate, silver sulfate and strontium sulfate are all insoluble. So I guess any of these cations could be in the solution and they can be precipitate B.

    Transition metal hydroxides are insoluble. So any transition metal hydroxide can be precipitate C.

    I think I got confused and thought that the Na2SO4 was added to precipitate A and not the solution. Are these the answers you are expecting?

    Thanks!
     
  5. Jun 30, 2016 #4

    Borek

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    Staff: Mentor

    Not bad, but not entirely correct either. One of these cations is for sure not present in the solution.

    White, gelatinous. Compounds of transition metals are rarely colorless.
     
  6. Jun 30, 2016 #5

    TT0

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    I would say lead because I guess it is less reactive than silver so the precipitate formed will be Ag2SO4 and not PbSO4. But this isn't really a logical explanation I think.

    I am not sure which transition metals produce colourless compounds. I am guessing the neutral element should be a silver colour so maybe zinc or iron?

    Cheers!
     
  7. Jul 1, 2016 #6

    Borek

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    Staff: Mentor

    What was the very first step of the procedure?

    Just try to google for "gelatinous white hydroxide".
     
  8. Jul 1, 2016 #7

    TT0

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    I see, the AgCl was already removed so no silver cations would be left. I misinterpreted the question. :(

    Zinc and aluminium hydroxides form this. They also dissolve in excess NaOH. Why does this happen? The reaction for zinc is:

    Zn2+ + 2NaOH ↔ Zn(OH)2 + 2Na+

    If more NaOH is added, then the equilibrium will shift to the right so why does it dissolve again in excess NaOH?

    Thanks!
     
  9. Jul 1, 2016 #8

    Borek

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    Staff: Mentor

  10. Jul 1, 2016 #9

    TT0

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    I see thanks! So the answer I gave above is correct?
     
  11. Jul 1, 2016 #10

    Borek

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    Staff: Mentor

    Yes.
     
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