Doc Brown's Chemistry  Theoretical Chemistry - Equilibria - Chemical Equilibrium 5.5

5.5 Definition, examples, pH & pK_w strong base calculations

Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry revising courses for pre-university students (equal to US grade 11 and grade 12 and Honours/honors level courses) KS4 Science GCSE/IGCSE Chemistry reversible reactions-equilibrium * KS4 Science GCSE/IGCSE notes acids and bases * KS4 Science GCSE/IGCSE notes acid-base theory

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-K_a-pK_a calculations * 5.5 strong bases-examples-pH calculations * 5.6 weak bases- examples & pH-K_b-pK_b calculations * 5.7 A level notes on Acids, Bases, Salts, uses of acid-base titrations - upgrade from GCSE! * EMAIL query?comment

Advanced Equilibrium Chemistry Notes Part 1. Equilibrium, Le Chatelier's Principle-rules * Part 2. K_c and K_p 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 pK_w 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 K_b (compare K_b 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.

    • K_w = [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 = pK_w - 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

    • K_w = [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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