pH of weak acid/weak base salts

[dumuzi]

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

1) Ca^2^+ + H_2O --> Ca(OH)_2 + H_3O^+

and

2) CO_3^2^- + H_2O --> HCO_3^- + OH^-

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

[GCT]

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).

[Nerro]

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 H3O^+.

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

The second reaction is also incorrect since it's an equilibrium:
CO_3^{2-}(aq) + H_2O(l) \rightleftharpoons HCO_3^-(aq) + OH^-(aq)

[Borek]

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

H_2CO_3
pKa1 = 6.37
pKa2 = 10.25

CaCO_3
pKso = 8.3

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

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 CaCO_3 (due to Ca^{2+} and CO_3^{2-} hydrolysis - although the first can be neglected, as Ca^{2+} is dominant at 99.9% of total calcium concentration).

Best,
Borek
--
Chemical calculator (http://www.chembuddy.com/?left=chemical_calculators)s at www.chembuddy.com (http://www.chembuddy.com)
pH calculation (http://www.chembuddy.com/?left=BATE&right=example_of_pH_calculation)
concentration conversion (http://www.chembuddy.com/?left=CASC&right=concentration_conversion)