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

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

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

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

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

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

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

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 = 2.4pH + pOH = 14pH + 2.4 = 14pH = 11.6

pH > 7 is basic11.6 > 7Ammonia solution is basic

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 = 7.45pH + pOH = 147.45+ pOH = 14pOH = 6.55

pH > 7 is basic7.45 > 7solution is basic

basic