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!
  1. Choose species that can produce H+ and write reactions
  2. Based on K values, decide on dominant equilibrium
  3. Write equilibrium expression for dominant equilibrium
  4. ICE Table
  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

  • 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