Calculating the pH of a Solution of a Weak Monoprotic Acid, HA(aq), Given the Value of Ka
Calculate the pH and [H+(aq)] of a 0.30 mol/L solution of butanoic acid, HC4H7O2(aq).
The Ka of butanoic acid is 1.52 x 10-5.
Step 1: List the major entities in solution. Write balanced equations for all entities that may produce H+(aq).
Step 2: Identify the dominant equilibrium, and write the equilibrium constant equation for the dominant equilibrium.
Step 3: Use the coefficients in the balanced equation of the dominant equilibrium to determine the changes in initial
concentrations that will occur as the dominant reaction proceeds to equilibrium. Record all values in an ICE table.
Step 4: Substitute the equilibrium concentrations of all entities (from the “Equilibrium” line in the ICE table) into the
acid ionization constant equation.
Step 5: Assume that [HA]initial – x  [HA]initial, but only if [HA]initial  100.
Ka
Step 6: Solve for x (which, for a monoprotic acid, equals [H+(aq)]).
Step 7: Use the 5% rule to validate any assumption made in step 5.
Step 8: Calculate pH from [H+(aq)] (the value of x).
Calculating the pH of a Solution of a Weak Base, Given the Value of Kb
Strychnine, C21H22N2O2(aq), is a weak base but a powerful poison. Calculate the pH of a 0.0010 mol/L
solution of strychnine. The Kb of strychnine is 1.0 x 10-6.
Step 1: List the major entities in solution. Write balanced equations for all entities that may produce OH-(aq).
Step 2: Identify the dominant equilibrium, and write the equilibrium constant equation for the dominant equilibrium.
Step 3: Use the coefficients in the balanced equation of the dominant equilibrium to determine the changes in initial
concentrations that will occur as the dominant reaction proceeds to equilibrium. Record all values in an ICE table.
Step 4: Substitute the equilibrium concentrations of all entities (from the “Equilibrium” line in the ICE table) into the
base ionization constant equation.
Step 5: Assume that [B]initial – x  [B]initial, but only if [B]initial  100.
Kb
Step 6: Solve for x (which equals [OH-(aq)]).
Step 7: Use the 5% rule to validate any assumption made in step 5.
Step 8: Calculate pOH from [OH-(aq)] (the value of x)
Step 9: Calculate the pH by substituting the value of pOH into the equation pH + pOH = 14.00.