Lesson 4.2 Calculating dissociation constants and concentrations

What is the ICE table method for equilibrium calculations?

I = Initial concentration, C = Change in concentration, E = Equilibrium concentration. Set up columns for each species and use stoichiometry to relate changes.

When can you assume [HA]initial ≈ [HA]equilibrium for weak acids?

When Ka is very small (Ka < 10⁻⁴), dissociation is minimal so the change in acid concentration is negligible. You MUST state this assumption when using it

How do you calculate Ka given initial concentration and pH?

Step 1: Find [H⁺] = 10⁻ᵖᴴ. Step 2: [A⁻] = [H⁺] (equal amounts formed). Step 3: [HA]eq ≈ [HA]initial (if Ka small). Step 4: Ka = [H⁺][A⁻]/[HA]

Worked example: Calculate Ka for 0.029 M CH₃COOH with pH 3.15

[H⁺] = 10⁻³·¹⁵ = 7.08 × 10⁻⁴ M. [CH₃COO⁻] = 7.08 × 10⁻⁴ M. [CH₃COOH]eq ≈ 0.029 M (assumption). Ka = (7.08 × 10⁻⁴)² / 0.029 = 1.8 × 10⁻⁵

How do you calculate pH from Ka and initial concentration?

Step 1: Set up ICE table with x = [H⁺] formed. Step 2: Ka = x² / [HA]initial (using assumption). Step 3: Solve for x. Step 4: pH = −log(x)

How do you calculate pOH and pH for a weak base using Kb?

Step 1: Use ICE table to find [OH⁻] = x. Step 2: Kb = x² / [B]initial. Step 3: Solve for x = [OH⁻]. Step 4: pOH = −log[OH⁻]. Step 5: pH = 14 − pOH

What assumption must you STATE in exam answers for weak acid/base calculations?

"Because Ka (or Kb) is very small, the dissociation is minimal, so [HA]equilibrium ≈ [HA]initial" — Always state this when you use the approximation

How are "hydronium ions" and "hydrogen ions" related?

They are used interchangeably. H₃O⁺ (hydronium) = H⁺ (hydrogen ion). Both represent the same species in aqueous solutio