Chemistry 40SUnit 5: Acids & BasesLesson 4

Learning Outcomes C12-5-08: Write the equilibrium

expression (Ka or Kb) from a balanced chemical equation.

C12-5-09: Use Ka or Kb to solve problems for pH, percent dissociation, and concentration.

Acid Dissociation Constant (Ka)

Strong acids completely dissociate do not establish equilibrium

Weak acids do not completely dissociate do establish equilibrium E.g. HA + H2O ⇌ H3O+ + A–

Can write an equilibrium law for weak acids

Acid Dissociation Constant (Ka)

HA + H2O ⇌ H3O+ + A–

Just like with Keq the value of Ka indicates the state of equilibrium The larger the Ka = the larger the [P]

stronger acid & more dissociation

Base Dissociation Constant Base Dissociation Constant (Kb)

reflects the strength of the base and its ability to dissociate

Everything works the same as with Ka…

Dissociation Constant ProblemsExample 1: A 0.10 mol/L solution of acetic acid is only partly ionized. If at equilibrium, the hydronium ion concentration is 1.3×10–3 mol/L, what is the acid dissociation constant, Ka?

Dissociation Constant ProblemsExample 2: HA is a weak acid with a Ka of 7.3×10–8. What is the concentration of all species (HA, H3O+ and A–) if the initial concentration of HA is 0.50 mol/L?

Percent Dissociation Several ways to describe acids and

bases: Concentration Strong or weak Degree of dissociation

Can also be described by percent dissociation:

x 100%

Percent Dissociation x 100% x 100%

Percent Dissociation ProblemsExample 3: Calculate the percent dissociation of a 0.100 mol/L solution of formic acid (HCH2O2) if the hydronium ion concentration is 4.21×10–4 mol/L.

Percent Dissociation ProblemsExample 4: Calculate the Kb of the hydrogen phosphate ion (HPO4

2–) if a 0.25 mol/L solution of sodium hydrogen phosphate is dissociated is 0.080%.

pH, pOH, Ka & Kb ProblemsExample 5: What is the pH of a 0.10 mol/L solution of the imaginary acid, HA, if its Ka is 4.0×10–9.

pH, pOH, Ka & Kb ProblemsExample 6: The pH of a 0.20 mol/L solution of CO3

2– ions, is 10.75. What is the Kb of carbonate ions?