Calculating the pH of Strong Bases
Using Kw to Work Out the pH of Strong Bases
Strong bases fully dissociate in solution to release (OH- ) ions. You can calculate the pH of a strong base using the Kw equation, as it sets up a relationship between [OH- ] and [H+].
Worked Example: Calculate the pH of a solution of 0.2 mol dm-³ of Ba(OH)2 at 25 °C
Answer:
Step 1: Calculate the concentration of hydroxide ions.
Two moles of hydroxide ions are released from each mole of barium hydroxide. The concentration of OH- ions = 0.2 x 2 = 0.4 mol dm-³
Step 2: Use the value of Kw to calculate the concentration of H+ ions
Step 3: Use the concentration of H+ ions to work out the pH.
Worked Example: Calculate the pH of the solution formed when 12.00 cm³ of 0.2 mol dm-³ hydrochloric acid is added to 24.00 cm³ of 0.2 mol dm-³ barium hydroxide solution at 298 K
Answer:
Step 1: Work out the moles of acid and base
Moles of acid = volume x concentration = 12/1000 0.2 = 2.4 x 10-³ mol
Moles of base = volume x concentration = 24/1000 x 0.2 = 4.8 x 10-³ mol
Step 2: Use the balanced equation to identify which substance is in excess
There are 2.4 x 10-³ moles of acid, so the amount of base reacting = 1.2 x 10-³ mol.
Therefore moles of base remaining in the solution = (4.8-1.2) x 10-³ = 3.6 x10-3 mol
Step 3: Work out the concentration of OH- ions remaining
Moles of barium hydroxide = 3.6 x 10-³
Therefore moles of OH- ions = 2 x 3.6 x 10-³ = 7.2 x 10-³ mol
Total volume in the solution = 24+ 12 = 36.00 cm³ = 0.036 dm³
Concentration of OH- ions = moles/ volume = 7.2 x 10-³/0.036 = 0.2 mol dm-³
Step 4: Use Kw to work out the H+ concentration
Step 5: Use [H+] to work out pH
