What is pH?.

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Presentation transcript:

What is pH?

Ion Product Constant for Water H2O(l)  H+(aq) + OH-(aq) Keq = Kw = [H+] x [OH-] = 1 x 10-14 The ion product constant for water (Kw) is the equilibrium constant expression for the self-ionization of water. The value is dependent upon temperature Kw = 1 x 10-14 at 298K

Ion Product Constant for Water Cont. From this you can calculate the hydrogen ion concentration or the hydroxide concentration. Keq = Kw = [H+] x [OH-] = 1 x 10-14 [H+] > [OH-] = acidic [H+] < [OH-] = basic [H+] = [OH-] = neutral

Calculations of [H+] or [OH-] Ex 1: At 298 K the H+ ion concentration of an aqueous solution is 1.0 x 10-5. What is the OH- ion concentration in the solution? Is the solution acidic, basic, or neutral? Kw = [H+] x [OH-] 1 x 10-14 = [1.0 x 10-5] x [OH-] 1.0 x 10-5 1.0 x 10-5 1.0 x 10 -9 = [OH-] [H+] & [OH-] comparison 1.0 x 10-5 > 1.0 x 10-9 0.00001 > 0.000000001 [H+] > [OH-] = solution is acidic

Calculations of [H+] or [OH-] Ex 2: At 298K the H+ ion concentration of an aqueous solution is 2.5 x 10-6. What is the OH- ion concentration in the solution? Is the solution acidic, basic, or neutral? Kw = [H+] x [OH-] 1 x 10-14 = [2.5 x 10-6] x [OH-] 2.5 x 10-6 2.5 x 10-6 4.0 x 10 -9 = [OH-] [H+] & [OH-] comparison 2.5 x 10-6 > 4.0 x 10-9 0.0000025 > 0.000000004 [H+] > [OH-] = solution is acidic

What is pH? The hydrogen concentration is cumbersome so Soren Sorensen came up with the pH scale. pH scale ranges from 0-14 pH <7 is acidic pH = 7 is neutral pH > 7 is basic

What is pH?

Calculating pH from [H+] We use logarithms to do this N = 10x, then log N = x pH = -log[H+] Ex3: What is the pH of a solution with a concentration of 1.0 x 10-6 hydrogen ions? Then tell if it is acidic, neutral or basic. pH = -log[H+] pH = -log[1.0 x 10-6 ] pH = 6.0 pH is acidic

Calculating pH from [H+] Cont. pH = -log[H+] Ex 4: What is the pH of a solution if the concentration of the hydroxide ions is 9.6 x 10-4 M? Then tell if it is acidic, neutral or basic. Kw = [H+] x [OH-] 1 x 10-14 = [H+] x [9.6 x 10-4] 9.6 x 10-4 9.6 x 10-4 1.0 x 10 -11 = [H+] pH = -log[H+] pH = -log[1.0 x 10-11 ] pH = 11 pH > 7 is basic

Calculating pOH pOH = -log[OH-] pOH = -log[4.0 x 10-3 ] pOH = 2.4 pOH = -log[OH-] pH + pOH = 14 Ex 5: Ammonia gas has a hydroxide-ion concentration of 4.0 x10-3 M. Calculate the pOH and pH at 298 K. Then tell if it is acidic, neutral or basic. pOH = -log[OH-] pOH = -log[4.0 x 10-3 ] pOH = 2.4 pH + pOH = 14 pH + 2.4 = 14 pH = 11.6 pH > 7 is basic 11.6 > 7 Ammonia solution is basic

Calculating pH and pOH pH = -log[H+] pH = -log[3.56 x 10-8 M] pOH = -log[OH-] or pH + pOH = 14 Ex 6: A solution contains a hydrogen ion concentration of 3.56 x 10-8 M. Calculate the pH and pOH at 298 K. Then tell if it is acidic, neutral or basic. pH = -log[H+] pH = -log[3.56 x 10-8 M] pH = 7.45 pH + pOH = 14 7.45+ pOH = 14 pOH = 6.55 basic pH > 7 is basic 7.45 > 7 solution is basic