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Homework Help: Will a precipitate form? Molar solubility question

  1. Mar 30, 2014 #1
    1. The problem statement, all variables and given/known data

    Hello all,
    This is a problem on a worksheet I was given and Im stumped!
    Will a ppt form when 35.00 mL of 1.0x10^-3 M CoSO4 is mixed with 15.00 mL of 7.50x10^-4 M Al2(SO4)3 and 200 ml of a buffer which is .200 M NH3 and .200 M NH4Cl?

    I do not need the answer, I just need a nudge in the right direction.

    2. Relevant equations

    Ksp Al(OH)3 = 5x10^-33
    Ksp Co(OH)2 = 3x10-16

    3. The attempt at a solution

    I'm not even sure where to start. I would usually write out the net ionic eqn in this situation then use an ICE table but I'm not sure even what the ionic equation will look like here. We didnt go over molar solubility that much in my lecture, so all the problems were relatively straight forward till I got to this one. There just seems to be a lot going on. Any help? Thanks in advance.
  2. jcsd
  3. Mar 31, 2014 #2
    The first step is to figure out what the OH concentration would be if none of the OH reacted with Al and Co. For that, you need to consider the equilibration of the reaction NH3+H2O=NH4 + OH. Don't forget that the two smaller solutions dilute the 200 ml solution to 260 ml.

  4. Apr 1, 2014 #3


    User Avatar

    Staff: Mentor

    Actually it is pretty simple - you just have to realize these are two problems, each of which can be solved separately.
  5. Apr 1, 2014 #4
    Thanks guys, I got it. Found [OH-] then just used ice tables/ Ksp values to see which would precipitate first. Just needed a nudge in the right direction. Turns out Co(OH)2 ppts
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