o Conclusion: At the equivalence point of the titration, unlike the titration of a strong acid and strong base, the pH is > 7. This is due to the production of the conjugate base of a weak acid. Example 9 (calculating K a ) Weak Base – Strong Acid Titration o Step 1 – A stoichiometry problem – reaction is assumed to run to completion – then determine remaining species. o Step 2 – An equilibrium problem – determine position of weak base equilibrium and calculate pH. Example 10 (weak base/strong acid titration) Titration of a Weak Base with a Strong Acid o In the case of a titration of a weak base, the process follows that of a weak acid in reverse. o There exists a region of buffering followed by a rapid drop in pH at the eq. point.
o In the case of a titration of a weak polyprotic acid (H n A) there are “n” equivalence points. o In the case of the diprotic oxalic acid, (H 2 C 2 O 4 ) there are two equivalence points. Acid – Base Indicator o Marks the end point of a titration by changing color o The equivalence point is not necessarily the same as the end point o You must choose an indicator that changes color in a range that includes the pH at the equivalence point Solubility Product o For solids dissolving to form aqueous solutions Bi 2 S 3 ( s ) ⇆ 2 Bi 3+ ( aq ) + 3 S 2- ( aq ) K sp = solubility product constant and K sp = [Bi 3+ ] 2 [S 2- ] 3 “Solubility” = s = concentration of Bi 2 S 3 that dissolves, which equals ½[Bi 3+ ] and 1/3[S 2- ] o Note: K sp is constant (at a given temperature) o s is variable (especially with a common ion present) Example 11 (calculating K sp