Unit 10 SUMMARY: Buffers & Acid-Base Titrations weak conjugate acid-base pair. resist pH changes (reacts added acid/base) buffer capacity: more moles of buffer can react more acid/base before significant pH changes. pH range: range of pH values buffer works. Optimum Capacity and Range: - weak acid with a pKa near desired pH - [HA] = [A–] (so pKa = pH)

Common Ion Effect: adding common ion shifts left causing the weak electrolyte to be less ionized. pH of Buffers: 0.12 M lactic acid, HC3H5O3 (Ka = 1.4  10−4) 0.11 M sodium lactate, NaC3H5O3 HC3H5O3 + H2O  H3O+ + C3H5O3– [H3O+] [C3H5O3−] [HC3H5O3] Ka = I 0.12 M 0 M 0.11 M C –x + x +x E 0.12 – x x 0.11 + x ≈ 0.12 M ≈ 0.11 M (x)(0.11) (0.12) 1.4  10−4 = x = 1.5  10−4 M H3O+ pH = –log(1.5  10−4) = 3.82

Titration The analytical technique used to calculate the moles in an unknown soln. mass % (g /gtotal) molarity (mol/L) molar mass (g/mol) buret titrant end point: indicator color change known vol. (V) known conc. (M) equivalence point: equal stoich. amounts (mols) react completely Safari Montage (2 min) Titration analyte known vol. (V) unknown conc. (M) (or moles)

(irrelevant conjugate) At Veq… moles added = moles reacted …the solution contains only water & salt. (irrelevant conjugate) SA with SB At Veq, pH = 7

WA with SB At Veq, pH > 7 (only water & conj. base) A– + H2O ↔ HA + OH– Animation: (http://www.chembio.uoguelph.ca/educmat/chm19104/chemtoons/chemtoons9.htm)

WB with SA At Veq, pH < 7 (only water & conj. acid) HA + H2O ↔ H3O+ + A– Indicators: weak acids with diff color conjugates choose one that changes color (has pKa) near pH of equivalence point (Veq) of titration.

WA with SB Verbally describe the visual curve: Weak acids: moderately low initial pH gradual pH rise (not flat then jump) subtle pH change at equiv. point (less steep) Weak bases: (same but “drop” not “rise”)

Titration Calculations Calculate unknown moles of the ANALYTE. (then calculate: M OR molar mass OR mass %) Stoich: (L X  mol X  mol Y) M X Calculate unknown VOLUME of TITRANT (mL) added to reach equivalence (all reacted). Stoich: (L X  mol X  mol Y  L Y) M X M Y Calculate unknown pH at any point in the titration (especially at equivalence). Stoich: What’s in the flask? Find excess H+ , OH–