1. Ionic Equilibria

2. Ionic Equilibria
Acid-Base
- Acid/ Base/ Salt/ Buffer
- Titration Curve/ Indicator
Solubility
- Dissolving
- Precipitation

3. Acid/ Base Equilibria

4. Ionic Equilibria
- only valid at 25⁰C
p = -log10
At 25°C:
pH + pOH = pKw = 14
pKa + pKb = 14
[H+][OH–] = 10-14
Ka × Kb = Kw = 10-14

5. Problem Solving
Approach
1. Determine what is in solution:
- Acid/ Base/ Salt/ Buffer
2. Apply the corresponding eqns

6. Acid/ Base
strong acid
weak acid
[H+] = [HA]
[H+] = Ka [HA]
pH = -log [HA]
pH = -log [H+]
strong base
weak base
[OH–] = [B]
[OH–] = Kb [B]
pOH = -log [B]
pOH = -log [OH–]

7. Salt
How to determine if salt is acidic/ basic/ neutral?
1.
2.
Split the salt into its ions.
Determine the parent acid/ base of the ions:
Acid
Base
Salt
Strong
Strong
Neutral
Strong
Weak
Acidic
conj acid of weak base hydrolyses H2O to form H+
Weak
Strong
Basic
conj base of weak acid hydrolyses H 2O to form OH–
Weak
Weak
Depends

8. Salt
Nature of salt
Formed from
Finding pH
Neutral
strong acid
+ strong base
=7
Acidic
strong acid +
weak base
treat as weak acid;
Basic
weak acid +
strong base
conj acid of weak base hydrolyses H2O
to form H+
use weak acid formula
treat as weak base;
conj base of weak acid hydrolyses H2O
to form OH–
use weak base formula

9. Salt
How to calculate pH of a salt solution:
[H3O+] = Ka × [salt]
[OH–] = Kb × [salt]
1.
Find Ka/ Kb of the ion from Kb/ Ka of the parent acid/ base  use Kw
2.
Find [salt] =
n (limiting reagent)
Vtotal
Vtotal = Vacid + Vbase

10. Buffer
Type
Formed from
pH
Acidic
weak acid +
conjugate base
<7
weak base +
conjugate acid
>7
Basic
pH = pKa + log
[salt]
[acid]
[salt]
pOH = pKb + log
[base]

11. Buffer
How to calculate pH of a buffer on adding small amounts of
H+ and OH–:
1. Determine which species is reacted and which is formed.
2. Calculate new amounts (in moles)
3. Substitute into buffer equation.

12. Buffer
1. When using buffer equation, just substitute no. of moles of salt
and acid/base since total volume is the same and cancels out.
nsalt/V
pH = pKa + log
nacid/V
2. Look out for [salt] = [acid] which simply means pH = pKa (same
for basic buffer)
3. When unsure if buffer is acidic or basic, look at pKa
e.g. HCO3–/CO32–
pKa = 10
since pKa is large, buffer is more basic than acidic
 use basic buffer formula

13. Titration Curve
There are three important points in a titration curve.
e.g. titration of weak acid against strong base

14. Titration Curve
Titration curve
What to observe What can be found
1
Initial pt
- pH
2
Equivalence pt
- pH
3
Half
equivalence pt
[H+] due to dissociation of
acid
[H+] due to hydrolysis of
salt;
- volume of base Given [base], [acid] can
be found and vice versa
- pH
pKa of acid

15. Titration Curve
Calculate pH at various points in titration curve:
1.
2.
3.
4.
At the beginning
In the middle (sometimes at half eq. pt)
At equivalence pt
Beyond eq. pt.
Strategy
1.
2.
Determine what is in solution
- Acid/ Base/ Salt/ Buffer
Use the relevant eqns

16. Titration Curve
e.g. titration of weak acid against strong base
Titration curve
What is in solution Equations
Initial pt
weak acid
Between initial acidic buffer
pt and
equivalence pt
At equivalence acidic salt
pt
Beyond
equivalence pt
strong base
pH = -log Ka × [acid]
[salt]
pH = pKa + log
[acid]
pH = -log Ka × [salt]
pOH = -log [B]