Chemistry 1011 TOPIC Acids and Bases TEXT REFERENCE

Chemistry 1011 TOPIC Acids and Bases TEXT REFERENCE Masterton and Hurley Chapter 4. 2 (Review), 13, 14. 1, 15. 1 (page 427), 21. 2 (page 589) Chemistry 1011 Slot 5 1

13. 3 p. H and p. OH YOU ARE EXPECTED TO BE ABLE TO: • Define p. H and p. OH. • Calculate the p. H and p. OH of a solution given the concentration of hydrogen ions or the concentration of hydroxide ions, and vice versa. • Identify strong acids and bases • Calculate the p. H and p. OH of solutions of strong acids and bases of known concentration. • Identify methods of measuring p. H Chemistry 1011 Slot 5 2

Defining p. H • The acidity or basicity of a solution can be described by the [H+] or [OH-] • These concentrations can vary from 10 -1 to 10 -14 • The p. H scale was proposed in order to provide an easier means of describing acidity p. H = -log 10 [H+] Chemistry 1011 Slot 5 3

Logarithms and p. H Examples • If a = bc, then logba = c • If a = 10 c, then log 10 a = c • If [H+] = 10 -1, then log 10 [H+] = -1 or -log 10 [H+] = 1 • What is the p. H of a solution with a hydrogen ion concentration of 10 -2 mol/L p. H = -log 10 [H+] = -log 10 [10 -2] = 2 Chemistry 1011 Slot 5 4

p. H Examples • What is the p. H of a water sample that has a hydrogen ion concentration of 4. 0 x 10 -5? p. H = -log 10 [H+] = -log 10 [4. 0 x 10 -5] = • What is the hydrogen ion concentration of a solution with a p. H of 5. 6? p. H = -log 10 [H+] 5. 6 = -log 10 [H+] = Chemistry 1011 Slot 5 5

p. OH • p. OH is determined in the same way as p. H p. OH = -log 10 [OH-] • Since KW = [H+]x[OH-] = 1. 0 x 10 -14 It follows that p. H + p. OH = 14 • If a solution has a p. H of 5. 6, then the p. OH will be 8. 4 • It will be basic Chemistry 1011 Slot 5 6

Interpreting the p. H Scale • In an acidic solution, [H+] > 1. 0 x 10 -7 [OH-] < 1. 0 x 10 -7 p. H 0 to 7 • In a basic solution, [H+] < 1. 0 x 10 -7 [OH-] > 1. 0 x 10 -7 p. H 7 to 14 Chemistry 1011 Slot 5 7

Calculating the p. H of a Strong Acid • Strong acids and bases are fully ionized in dilute aqueous solution HCl(aq) H+(aq) + Cl-(aq) Na. OH(aq) Na+(aq) + OH-(aq) • The [H+] or [OH-] can be determined from the [acid] or [base] Chemistry 1011 Slot 5 8

Example - Calculating the p. H of a Strong Acid • What is the p. H of a 2. 5 x 10 -2 mol/L solution of sulfuric acid, H 2 SO 4(aq)? H 2 SO 4 is a strong acid H 2 SO 4 2 H+(aq) + SO 42 -(aq) [H+] = 2 x 2. 5 x 10 -2 mol/L p. H = -log 10 [H+] = -log 10 [5. 0 x 10 -2] = Chemistry 1011 Slot 5 9

Example - Calculating the p. H of a Strong Base • What is the p. H of a 5. 0 x 10 -2 mol/L solution of barium hydroxide, Ba(OH)2(aq)? Ba(OH)2 is a strong base Ba(OH)2 Ba 2+(aq) + 2 OH-(aq) [OH-] = 2 x 5. 0 x 10 -2 mol/L p. OH = -log 10 [OH-] = -log 10 [1. 0 x 10 -1] = 1 p. H = 14 - p. OH = 13 Chemistry 1011 Slot 5 10

p. H of Common Materials • • Lemon Juice Vinegar Tomato Juice Cheese Cow’s Milk Human Saliva Human Blood Seawater 2. 2 - 2. . 4 3. 0 4. 8 - 6. 4 6. 3 - 6. 6 6. 5 - 7. 5 7. 3 - 7. 5 8. 3 Chemistry 1011 Slot 5 11

Measuring p. H • p. H Meter • Acid-base indicators – fruit juices – litmus – universal indicator (a mixture of different indicators that change colour at different p. H’s) – indicator paper Chemistry 1011 Slot 5 12