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Determining Percent Dissociation in Weak Acids and Bases

Learn how to calculate the percentage dissociation of weak acids like acetic acid using the dissociation equation and the equilibrium constant Ka. Understand the significance of percent dissociation and its relationship to initial concentrations.

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Determining Percent Dissociation in Weak Acids and Bases

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  1. Percent Dissociation Chapter 16 part V

  2. Percent Dissociation • This is the method to determine just how much of a weak acid or weak base dissociates or ionizes. • % dissociation=([dissociated]/[initial amount])X100 • Previously we found in 1.00 HF • that [H+]=2.7 X10-2 • What is % dissociate? • ((2.7 X10-2)/1.00)X100=2.7%

  3. Percent Dissociation • Example: • Find the % Dissociate of Acetic Acid (Ka =1.8 X 10-5) in the following two examples. • A. 1.00 M Acetic Acid • B. 0.10M Acetic Acid

  4. First: Major species • Equations • Ka Expression • I • C • E

  5. Answer • Acetic Acid Ka= 1.8 X 10-5 • H2O Kw = 1.0 X 10-14 • Acetic Acid wins! • HC2H3O2 H+ + C2H3O2- • I 1.00 0 0 • C -X +X +X • E 1.00-X X X • Ka= 1.8X10-5 = [H+][C2H3O2-] =X2/(1.00-X) • [HC2H3O2]

  6. Answer • X2/(1.00-X) ≈ X2/(1.00)= 1.8 X 10-5 • Why? X is insignificant relative to the value 1, • But it IS significant relative to Zero, so we keep the X2. • X2 =1.8 X 10-5 • X = 4.2 X 10-3 • Valid? • (0.0042/1.00)X100 = 0.42 % 5% rule • % dissociation ([H+]/[HC2H3O2])X100=0.42%

  7. Part B • Major species • Equations • Ka Expression • I • C • E

  8. Answer • HC2H3O2 H+ + C2H3O2- • I 0.10 0 0 • C -X +X +X • E 0.10-X X X • Ka= 1.8X10-5 = [H+][C2H3O2-] =X2/(0.10-X) • [HC2H3O2]

  9. Answer • X2/(0.10-X) ≈ X2/(0.10)= 1.8 X 10-5 • X = 1.3 X 10-3 • Still Valid and % Dissociation • (1.3 X 10-3/0.10)X100= 1.3% • The duh Factor • As the initial []’s of a weak acid is smaller, the % dissociation gets larger. • We can also find the Ka from % dissociation.

  10. Ka from % dissociation

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