1. ACIDS AND BASES:
Dissociation Constants

2. C12-5-09 ACID/BASE EQUILIBRIUM
OUTCOME QUESTION(S):
C ACID/BASE EQUILIBRIUM
Describe the relationship between the hydronium and hydroxide ion concentrations in water
Include: the ion product of water, Kw
Write the equilibrium expression (Ka or Kb) and solve problems involving pH, pOH, and percent dissociation.
These problem types use skills seen during the equilibrium unit and should be recognizable…
Vocabulary & Concepts 

3. HA H+(aq) + A–(aq) HA + H2O H3O+ + A- BOH B+(aq) + OH–(aq)
Strong Acid
HA + H2O H3O+ + A-
Weak Acid
Ka =
[H+][A-]
[HA]
Acid dissociation constant:
BOH B+(aq) + OH–(aq)
Strong Base
B + H2O BH+ + OH-
Weak Base
Kb =
[B+][OH-]
[B]
Base dissociation constant:

4. Ka = 1.7 x 10-5 HC2H3O2 + H2O H3O+ + C2H3O2- [I] [C]
Initially a 0.10 M solution of acetic acid, it reaches equilibrium with a [H3O+] = 1.3 x 10-3 M. What is the acid dissociation constant, Ka?
ICE table…
HC2H3O2 + H2O H3O+ + C2H3O2-
[I]
[C]
-1.3 x x x 10-3
x 10-3
[E]
1.3 x 10-3
Ka =
[H3O+][C2H3O2-]
[HC2H3O2]
Ka = 1.7 x 10-5
Ignore the units for K
Ka =
[1.3 x 10-3][1.3 x 10-3]
[0.0987]

5. HA(aq) + H2O(l) H3O+(aq) + A-(aq)
The weak acid HA has a [HA]i = 0.50 M and an equilibrium constant of Ka = 7.3 x 10-8.
What are the equilibrium concentrations?
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
[I]
-x x x
[C]
[E]
0.5 - x x x
Type IV Problem:
Given only initial values and Keq
Solve for the variable x
Ka =
[H+][A-]
[HA]

6. HA(aq) + H2O(l) H3O+(aq) + A-(aq)
0.5 - x x x
[E]
x x 10-4
~ 0.5
*see…it was OK to ignore it
Ka =
[H+][A-]
[HA]
7.3 x = [x][x]
0.50
Based on the very small Ka value – 7.3 x 10-8 – assume that x will be negligible compared to the large 0.50 initial concentration and ignore it (always)
- x
(7.3 x 10-8)(0.50) = x2 √
3.65 x = x2 √
1.9 x 10-4 = x

7. If you can’t make the equation – find it on the Acid Strength Chart
Calculate the pH of a 0.10 mol/L hydrogen sulfide solution. (Ka=1.0 x 10-7)
If you can’t make the equation – find it on the Acid Strength Chart
H2S(g) + H2O(l) H3O+(aq) + HS-(aq)
0.10-x x x
[E]
[I]
1.0 x 10-4
[C]
-x x x
Ka =
[H+][HS-]
[H2S] √
1.0 x = x2 √
[E]
0.10-x x x
1.0 x 10-4 = x
1.0 x = [x][x]
0.10
Equation 1:
pH = - log [H+]
= - log(1.0 x 10-4)
= 4.00
- x
(1.0 x 10-7)(0.10) = x2

8. How much product has formed
Percent Dissociation
Ka / Kb represent the degree of dissociation
How much product has formed
Percent dissociation (PD) is another way to represent how much has ionized / dissociated in solution (acidic or basic):
Percent dissociation = [H+]
[HA]
• 100%
Percent dissociation = [OH-]
[B]
• 100%

9. Weak acid <<100% dissociated
Calculate the PD of a solution of formic acid (CH2OOH) if the hydronium ion concentration is .
0.100 M
4.21 x 10-3 M
CH2O2H + H2O H3O+ + CH2O2-
[E] x x 10-3
Percent dissociation = [H+]
[HA]
• 100%
Weak acid <<100% dissociated
PD = [4.21 x 10-3]
[0.100]
• 100%
PD = 4.21% of the original acid dissociated

10. HPO42- + H2O H2PO4- + OH- [E] 0.25 - x +x +x
Calculate the Kb and pH of a 0.25 M solution of a hydrogen phosphate if the dihydrogen phosphate ion (H2PO42-) dissociates 0.080%.
1. Construct the solution equation H+
HPO H2O
H2PO OH-
base
[E] x x x
Unknown Equation
Find the compound structure
Figure out if the solution is acidic or basic
Move the proton and create the products
Fill in the necessary ICE table information
You may not need to write the ICE table down if you recognize the pattern…

11. This is the kind of compound question to expect on a test…
HPO H2O H2PO OH-
[E] x x
- x
2.0 x 10-4
2.0 x 10-4
2. Use the PD formula to complete the ICE table
PD = [OH-]
100% •
[B]
(0.080%)(0.25) = [OH-]
100%
[OH-]= 2.0 x 10-4 M

12. HPO42- + H2O H2PO4- + OH- Kb = [H2PO4-][OH-] [HPO42-]
3. Use ICE table to find Kb and pH
HPO H2O H2PO OH-
[E] x x
2.0 x 10-4
2.0 x 10-4
Kb = [H2PO4-][OH-]
[HPO42-] pH
pOH
[OH-]
Kb= [2.0 x 10-4][2.0 x 10-4]
0.25
Equation 3:
pOH = -log[OH-]
pOH = 4.00
Equation 5:
pH =
pH = 10.00
Kb = 1.6 x 10-7
Be careful with pH – the solution is basic so we have to make conversions

13. CAN YOU / HAVE YOU?
C ACID/BASE EQUILIBRIUM
Describe the relationship between the hydronium and hydroxide ion concentrations in water
Include: the ion product of water, Kw
Write the equilibrium expression (Ka or Kb) and solve problems involving pH, pOH, and percent dissociation.
Vocabulary & Concepts