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Homework Help: PH of water in limestone

  1. Dec 11, 2009 #1
    1. The problem statement, all variables and given/known data
    Find the pH of water in limestone (CaCO3) quarry.
    H2CO3 pk1=6.37, pk2=10.32

    2. Relevant equations
    pH=pKa + log [conjugate base]/[acid]
    Kw=10^-14

    3. The attempt at a solution
    CO3 2- + H2O -> HCO3- + OH-
    Ka= [HCO3-][OH-]/[CO3 2-]
    10^-10.32=x^2/[10^-6.37]
    [OH-]=x=.01059
    -log[OH-]=1.98
    14-1.98=12.02
    is this correct?
     
    Last edited: Dec 11, 2009
  2. jcsd
  3. Dec 11, 2009 #2
    since its a salt, I think you can do pH= 1/2 (pKa1 + pKa2)
     
  4. Dec 12, 2009 #3

    Borek

    User Avatar

    Staff: Mentor

    No, that would work for solution of amphiprotic salt, here you start with fully neutralized carbonate.

    A lot depends on the initial cncentration of carbonates - and this depends on the calcium carbonate solubility product. Besides, solubility will increase due to carbonates hydrolyzis, so finding an exact answer can be tricky. No idea how deep you are expected to get into it.

    Why do you think [CO32-] = Ka1?

    --
    methods
     
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