PH of weak acid/weak base salts

  • Thread starter dumuzi
  • Start date
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]


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


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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).
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]


pKb1 = 2.43
pKb2 = 1.40
( [Broken])

pKa1 = 6.37
pKa2 = 10.25

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 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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