Acid Dissociation Constant
- For weak acids only:
(note [H+] is the same as [H3O+])
○ Strong acids don’t have Ka
- Larger Ka = stronger weak acid → will produce more H3O+ ions and thus a lower (more acidic) pH
Weak Acids
- Any acid that is not one of the 6
- They do not dissociate completely
- Equilibrium far to the left
- [H+ ] <<< [HA]
- A- is a stronger base than water
Strong vs Weak Acids
- If acid is very weak → conj. base is very strong
- If acid is very strong → conj. base is very weak
- Amount of H3O+ indicates how strong an acid is
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
- Water will always be a major species in pH calculations!
- Choose species that can produce H+ and write reactions
- Based on K values, decide on dominant equilibrium
- Write equilibrium expression for dominant equilibrium
- ICE Table
- List initial concentrations in dominant equilibrium
- Define change at equilibrium (as X)
- Write equilibrium concentrations in terms of x
- Plug in equilibrium concentrations into eq expression
- Find the intersection between the eq expression and Ka value graphically
- Calculate [H+] and pH
Practice Notes
- Assume that
Weak Bases
- 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
○ “amount dissociated” is the equilibrium concentration of H+
- 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
Polyprotic Acids
- A polyprotic acid always gives up its protons one reaction at a time
○ Ex: 
- In calculations, focus attention & write an ICE table for only the first equation Ka value bcuz is much larger (ie. donates most of the H+)
- “Species in lowest concentration” will be the one in the last step