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Acid-Base Equilibria Lecture: Weak Acid Problem Example

Understand pH and major species concentrations in formic acid solution using systematic and ICE methods with helpful examples. Learn when assumptions are valid and how to solve weak base problems. Explore buffer solutions and calculate pH for specific scenarios.

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Acid-Base Equilibria Lecture: Weak Acid Problem Example

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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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