Page 75 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

Equilibrium 11 59

POLYPROTIC ACIDS
Polyprotic acids are acids which have more than one replaceable
proton. Examples include H2SO4 (eA = 2), H3P04 (eA = 3), etc.,
where eA is the number of replaceable hydrogens. Therefore, sulfuric
acid, H2S04, has two dissociation constants, Ka(1) and Ka(2) respec-
tively, corresponding to the following reactions:

H2SO4 + H20 + H30+ + HSO,, where Ka(1) = {[H30'][HSO,])/
[H2SO4] = 1.3 x and HSO; + H20 + H30f + SO:-, where
Ka(2) = {[H3O'][SO:-])/[HSO,] = 6.3 x . AS expected, Ka(2)
= 6.3 x < Ka(l) = 1.3 x lov2 , since HSO; is a relatively weak
acid and is negatively charged.

SOLUBILITY PRODUCT
In precipitation reactions, slightly soluble products often form, and
there is an equilibrium between the solid and the ions of the saturated
solution, e.g. for the reaction AgCl,,) + Ag+(aq) + Cl-(aq), the
forward reaction is a dissolution reaction, and the back reaction is a
precipitation reaction + K, = { [Ag + (aq)][Cl-(aq)])/[AgCl(s)]. But,
since the activity, a, of a solid is unity, this means the equilibrium
constant can be expressed as Ksp = [Ag+(aq)][Cl-(aq)], where Ksp is
termed the solubility product. Likewise for the reaction Mg(OH),,,) +
Mg2+(aq) + 20H-(aq), Ksp = [Mg2+][OH-I2, etc.

Solubility Product Equilibrium Problems

Example No. 1: The solubility product of calomel, Hg2C12 is 1.1 x
I 10-l8. Determine the molar solubility of Hg2C12.
~
Solution:
1. Solubility product question!
2. Hg2Cl2,,) i= HgZ,+(aq) + 2C1-(aq)-Heterogeneous equilibrium (2
phases).
3. ~sp [~g:+(aq~cl-(aq)~~-
=
4. Hg2C12,s) 'C H&+(aq) + 2Cl-(aq)
Initial conc. 0 0
Change +X + 2x
Final conc. +X + 2x