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Theoretical–Physical Advanced Level Chemistry – Equilibria – Chemical Equilibrium Revision Notes PART 5.5

5.5 Definition of a strong base and examples of pH & pKw strong base calculations

What is a strong base? How to calculate the pH of a strong base solution using Kw given the acid concentration.

(c) doc b GCSE/IGCSE reversible reactions–equilibrium notes

and (c) doc b GCSE/IGCSE notes on acids and bases

Equilibria Part 5 sub–index:  5.1 Lewis and Bronsted–Lowry acid–base theories * 5.2 self–ionisation of water and pH scale * 5.3 strong acids–examples–calculations * 5.4 weak acids–examples & pH–Ka–pKa calculations * 5.5 strong bases–examples–pH calculations * 5.6 weak bases– examples & pH–Kb–pKb calculations * 5.7 A level notes on Acids, Bases, Salts, uses of acid–base titrations – upgrade from GCSE!

Advanced Equilibrium Chemistry Notes Part 1. Equilibrium, Le Chatelier's Principle–rules * Part 2. Kc and Kp equilibrium expressions and calculations * Part 3. Equilibria and industrial processes * 4. Partition, solubility product and ion–exchange * Part 5. pH, weak–strong acid–base theory and calculations * Part 6. Salt hydrolysis, Acid–base titrations–indicators, pH curves and buffers * Part 7. Redox equilibria, half–cell electrode potentials, electrolysis and electrochemical series * Part 8. Phase equilibria–vapour pressure, boiling point and intermolecular forces


5.5 Definition, examples and pH and pKw calculations of strong bases

  • 5.5.1: Definition and examples of STRONG BASES

    • Strong bases are highly ionized in water.

    • 100% ionisation is assumed in the calculations and in reality strong bases have a very high Kb (compare Kb for weak bases).

    • Examples:

      • Gp 1 hydroxides e.g. sodium hydroxide: NaOH(s) + aq ==> Na+(aq) + OH(aq)

      • Gp 2 hydroxides e.g. barium hydroxide: Ba(OH)2(aq) + aq ==> Ba2+(aq) + 2OH(aq)

    • The high % of ionisation gives the maximum concentration of hydroxide ions and the minimum equilibrium concentration of hydrogen ions, therefore the most alkaline solutions of highest possible pH.

  • 5.5.2: Strong base calculations – calculating the pH of a strong base

    • Calculation example 5.5.2a

      • Calculate the expected hydroxide and hydrogen ion concentrations and pH of a 0.50 mol dm–3 sodium hydroxide solution.

      • [OH(aq)] = [NaOH(aq)] = 0.50 mol dm–3

      • In base calculations you need to use the ionic product of water expression to calculate the H+ ion concentration.

      • Kw = [H+(aq)] [OH(aq)] = 1 x 10–14 mol2 dm–6 and rearranging gives

      • [H+(aq)] = Kw/[OH(aq)] = 1 x 10–14/0.50 = 2.0 x 10–14 mol dm–3

      • pH = –log(2.0 x 10–14) = 13.7

      • Note: pOH = pKw – pH = 14 – 13.7 = 0.3

    • Calculation example 5.5.2b

      • Calculate the expected hydroxide and hydrogen ion concentrations and pH of a 0.02 mol dm–3 calcium hydroxide solution.

      • [OH(aq)] = 2 x [Ca(OH)2(aq)] = 0.04 mol dm–3

      • Kw = [H+(aq)] [OH(aq)] = 1 x 10–14 mol2 dm–6 and rearranging gives

      • [H+(aq)] = Kw/[OH(aq)] = 1 x 10–14/0.04 = 2.50 x 10–13 mol dm–3

      • pH = –log(2.50 x 10–13) = 12.6

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Equilibria Part 5 sub–index:  5.1 Lewis and Bronsted–Lowry acid–base theories * 5.2 self–ionisation of water and pH scale * 5.3 strong acids–examples–calculations * 5.4 weak acids–examples & pH–Ka–pKa calculations * 5.5 strong bases–examples–pH calculations * 5.6 weak bases– examples & pH–Kb–pKb calculations * 5.7 A level notes on Acids, Bases, Salts, uses of acid–base titrations – upgrade from GCSE!

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