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Lecture

Lecture. Acid – Base Equilibria. Weak Acid Problem Example: What is pH and the concentration of major species in a 2.0 x 10 -4 M HCO 2 H (formic acid, K a = 1.80 x 10 -4 ) solution ? Approaches: Systematic Method: too complicated for a “complete” solution

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Lecture

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  1. Lecture

  2. Acid – Base Equilibria Weak Acid Problem Example: What is pH and the concentration of major species in a 2.0 x 10-4 M HCO2H (formic acid, Ka = 1.80 x 10-4) solution ? Approaches: Systematic Method: too complicated for a “complete” solution with assumptions in charge balance equation ([H+] = [A-] + [OH-] goes to either [H+] = [A-] or [H+] = [OH-]), problem can be solved easily ICE Method can be used in “most cases” (or specifically when the charged balance equation can be simplified to [H+] = [A-])

  3. Acid – Base Equilibria Weak Acid Problem – Example Results x = 1.2 x 10-4 M = [H+] = [A-] (using either systematic method with assumption or ICE method) Note: we know assumption [H+] = [A-] is valid since [OH-] = Kw/[H+] = 8 x 10-11 M << [A-] [HA] = [HA]o – [A-] = 2.0 x 10 M - 1.2 x 10-4 M [HA] = 8 x 10-5 M Fraction of Dissociation = a = [A-]/[HA]total a = 1.2 x 10-4 /2.0 x 10-4 = 0.60

  4. Acid – Base Equilibria Weak Acid Problem – cont.: When is Assumption #1 ([H+] = [A-]) valid (in general)? When both [HA]o and Ka are high or so long as [H+] > 10-6 M More precisely, when [HA]o > 10-6 M and Ka[HA]o > 10-12 See chart (shows region where error < 1%) Assupmption #1 Works Fails

  5. Acid – Base Equilibria Weak Base Problem: As with weak acid problem, ICE approach can generally be used (except when [OH-] from base is not much more than [OH-] from water) Note: when using ICE method, must have correct reaction Example: Determine pH of 0.010 M NH3 solution (Ka(NH4+) = 5.7 x 10-10, so Kb = Kw/Ka = 1.75 x 10-5) Reaction NH3 + H2O  NH4+ + OH- You can do the rest For bases, a = fraction of dissociation = [BH+]/[B]o

  6. Acid – Base Equilibria Buffer Solutions: A buffer solution is designed so that a small addition of acid or base will only slightly change the pH Most buffer solutions have a weak acid and its conjugate base both present Example: Determine pH of a mix of 0.010 M HCO2H and 0.025 M Na+HCO2- solution (ignoring activity) Go to board to show if ICE approach is needed

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