AP Chemistry Unit 8.3 Weak Acid and Base Equilibria

Acid Dissociation Constant

(only) For weak acids: (note [H+] is the same as [H3O+])
- Strong acids don’t have Ka

Weak Acids

Strong vs Weak Acids

Process to Calculate pH of (weak acid) Solutions

- Don’t memorize → instead know the chemistry bcuz there is no ONE way to do these problems

1. List major species in solution

2. Choose species that can produce H+ and write reactions

- Will sometimes have to first write out net ionic equation → will reveal another equation that produces [H+] or [OH-]

3. Based on K values, decide on dominant equilibrium

4. Write equilibrium expression for dominant equilibrium

5. List initial concentrations in dominant equilibrium

6. Define change at equilibrium (as X)

7. Write equilibrium concentrations in terms of x

8. Plug in equilibrium concentrations into eq expression

9. Find the intersection between the eq expression and Ka value graphically

10. Calculate [H+] and pH

Practice Notes

Weak Acids

Ka (given) will be small → a weaker acid will produce fewer H3O+ ions and thus a higher (less acidic) pH
Always write down the major species! → then use ICE Tables
Now x is the [OH-] → easier to find the pOH first

Percent Dissociation

- 100% for strong acids
- Numerator = x; denominator = []o /
For a weak acid, percent dissociation increases as an acid becomes more dilute
- As [HA]₀ decreases, [H+] decreases but % dissociation increases
Note: Percent dissociation → acids; Percent ionization → bases
- Are set up the same way

Summary

Polyprotic Acids

A polyprotic acid always dissociates in a stepwise manner, one proton at a time
- Ex:
In calculations, can normally ignore the 2nd (& sometimes third) dissociations bcuz the first is much larger (ie. donates most of the H+)