Ch 16: pH of Mixtures In the last chapter, we looked at pH of mixtures of only acids or only bases. This chapter looks at other types of mixtures A or B with salt that has conjugate (buffers) SA with SB WA with SB or WB with SA titrations

pH of A or B with conjugate salt Common Ion Effect = Buffer In the last unit, we found the pH of a 1.0 M solution of HF to be 1.57. HF + H2O H3O+ + F- Would you expect the pH of the solution to change if some NaF were added? Would it increase, decrease, or stay the same and why? Two ways to explain this…(one is LeChat.) Would you expect the % dissociation to change or stay same? Explain.

Calc pH and % dissoc of HF with common ion What is pH and % dissoc of a solution that contains 1.0 M HF and 1.0 M NaF? Ka HF = 7.2x10-4 pH of HF alone = 1.57 % dissoc = 2.7%

Buffer Solution WA plus salt with CB WB plus salt with CA WA with LR SB; WB with LR SA Resist change in pH when small amounts of additional acid or base are added Optimal buffer has [WA] = [CB] or [WB] = [CA]. In this case, pH of the buffer = pKa of the acid. The buffer range/capacity will be  1 pKa.

Problem to Try How to prepare a buffer of acetic acid, CH3CO2H… Identify acetic acid as SA, SB, WA, WB What salt could you add to make a buffer? How much salt to add to make best buffer? If Ka = 1.8x10-5 and you prepare an ideal buffer, what pH range would this buffer work?

Another Problem NH3 Kb = 1.8 x 10-5 CH3NH2 Kb = 4.38x10-4 C5H5N Kb = 1.7x10-9 Are these SA, SB, WA, WB? Salt to make a buffer? Which one would make an ideal buffer with pH = 5?

pH if mix A with B = Titration Identify if mixture is SA with SB, WA with SB, or WB with SA Identify equivalence point Do calculations based on where you are in the titration relative to the equivalence point Cheat sheets

Do intro titration handout to review definitions and to revisit calculations for SA/SB titration Then, do weak with strong titration

SA/SB Titration Problem Titrate 10.0 mL of 0.100 M HCl with 0.100 M NaOH. Identify the analyte and titrant. How many mL of NaOH to reach equiv pt? Initial pH of the HCl solution? If add 2.00 mL of the NaOH, what is the pH? pH at equivalence point? pH if add 12.0 mL of NaOH?