1. Determination of Ka, Kb & pH

2. Solving Acid/Base Problems
1. List the major species in solution
eg. HA, A-, B:, HB+, H3O+, OH-
Look for reactions that are not equilibrium systems.
eg. HCl H2O  H3O Cl-
NaOH  Na+ + OH-
NaC2H3O2  Na+ + C2H3O2-
3. For above, determine [H3O+] or [OH-] or [A-].

3. 4. Determine the equilibrium systems and pick the equilibrium that will control the pH.
eg. Ka > Kb > Kw or Ka1 > Ka2
Calculate the [H3O+]i or [OH-]i or [A-]I from [HA]i or [B:]i and the values from #3 above.
Write the equation for the rxn and the Ka or Kb.
Create an I.C.E. table, calculate 100 Rule.
Substitute [ ]eq into Ka or Kb.
9. Solve for the pH or [ ]eq or Ka or Kb as required.

4. 1) Determining Ka or Kb From pH
eg. Calculate the Ka for formic acid (HCO2H) if a M solution has a pH of 2.38
HCO2H + H2O  H3O+ + CO2H- ;
ICE
0.100
- x +x +x
[H3O+]eq = x = 10-pH = =
= 1.8 x 10-4

5. From p different determination of [H3O+]eq or [OH-]eq.
eg. Calculate the Kb for NH3 if a M solution has a p of 7.8 %.
NH H2O  NH OH- ;
ICE x +x +x
[OH-]eq = x = [NH4+]eq
= ;
[NH3]eq
= 0.140
= 1.0 x 10-3

6. 2) Determining pH from Ka or Kb
Development of the ‘Shortcut’
Given:
HA H2O  H3O A- ;
ICE
[HA]i – x +x +x
If 100 Rule applies, then:
then
and
also

7. eg. Calculate the pH of a 0.200 M acetic acid sol’n.
Let HA = HC2H3O2 = acetic acid
HA H2O  H3O A- ;
ICE
0.100 – x +x +x
= 1.8 x 10-5
100 Rule 
= 2.87

8. 3) Polyprotic Acids ionize only 1 H+ at a time.
each step has its own Ka, designated Ka1, Ka2, etc.
Ka1 is the strongest and sets the pH of the sol’n.
eg. Calculate the pH of M H2CO3 and the [H2CO3]eq, [HCO3-]eq, [CO32-]eq.

9. eg. Calculate the pH of 2.500 M H2CO3 and the [H2CO3]eq, [HCO3-]eq, [CO32-]eq.
H2CO H2O  H3O+ + HCO3- ;
ICE
2.500 – x +x +x
100 Rule 
= 4.4 x 10-7
= 1.0 x 10-3
[H2CO3] =
= 2.499
pH = -log (1.0 x 10-3)
= 3.00

10. eg. Calculate the pH of 2.500 M H2CO3 and the [H2CO3]eq, [HCO3-]eq, [CO32-]eq.
HCO H2O  H3O CO32- ;
ICE
1.0 x 10-3 – x
1.0 x x
+ x
100 Rule 
= 4.7 x 10-11
so [HCO3- ] = [H3O+ ],
Always if 100 Rule applies or
= 4.7 x 10-11