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PH of weak acid/weak base salts

  1. Jul 25, 2005 #1
    I'm trying to show my students that [itex]CaCO_3[/itex] is basic.
    [itex]Ca^2^+[/itex] comes from [itex]Ca(OH)_2[/itex], which is a weak base due to it's low solubility. [itex]CO_3^2^-[/itex] comes from the weak acid [itex]HCO_3^-[/itex]. To compare the two we compare:

    1) [itex]Ca^2^+ + H_2O --> Ca(OH)_2 + H_3O^+[/itex]

    and

    2) [itex]CO_3^2^- + H_2O --> HCO_3^- + OH^-[/itex]

    We want to compare the K values. It is clear that equation 2 will have a significantly larger K value and the solution will be basic, but where can a person find a K value for something like equation 1?
    Are there charts for the Ka of metal ions?
    What order of magnitude is it? I imagine Ka for 1 is <<< Kw
     
  2. jcsd
  3. Jul 25, 2005 #2

    GCT

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    Science Advisor
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    I don't think equation 1 will occur appreciably, calcium hydroxide is fairly soluble in water so you can reasonably neglect it; thus calcium is a spectator ion (solubility of calcium hydroxide is .18g/100mL).
     
  4. Jul 25, 2005 #3
    The first reaction isn't even correct and frankly I don't know why you think that a strong base like calciumhydroxide would create an acidic particle like [tex]H3O^+[/tex].

    This reaction would occur:
    [tex]Ca(H_2O)_6^{2+}(aq)+H_2O(l) \rightleftharpoons CaOH(H_2O)_5^+(aq)+H_3O^+(aq)[/tex] Which explains why a solution of [tex]CaCl_2[/tex] is acidic.

    The second reaction is also incorrect since it's an equilibrium:
    [tex]CO_3^{2-}(aq) + H_2O(l) \rightleftharpoons HCO_3^-(aq) + OH^-(aq)[/tex]
     
  5. Jul 25, 2005 #4

    Borek

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    [tex]Ca(OH)_2[/tex]
    pKb1 = 2.43
    pKb2 = 1.40
    (http://ifs.massey.ac.nz/resources/chemistry/dissociation/inorgbases.htm)

    [tex]H_2CO_3[/tex]
    pKa1 = 6.37
    pKa2 = 10.25

    [tex]CaCO_3[/tex]
    pKso = 8.3

    So the concentration of saturated [tex]CaCO_3[/tex] solution is [tex]7.1\cdot10^{-5}[/tex]

    pH calculated using BATE is 9.73.

    (see http://www.chembuddy.com/?left=pH-calculation&right=pH-salt-solution for general approach to pH calculation of salt solution).

    In fact final pH will be different, as you should take into account increased solubility of [tex]CaCO_3[/tex] (due to [tex]Ca^{2+}[/tex] and [tex]CO_3^{2-}[/tex] hydrolysis - although the first can be neglected, as [tex]Ca^{2+}[/tex] is dominant at 99.9% of total calcium concentration).
     
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