1. What is pH?

2. 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 at 298K

3. 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

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

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

6. 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

7. What is pH?

8. 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

9. 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 = [H+] x [9.6 x 10-4]
9.6 x x 10-4
1.0 x = [H+]
pH = -log[H+]
pH = -log[1.0 x ]
pH = 11
pH > 7 is basic

10. 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 = 14
pH = 11.6
pH > 7 is basic
11.6 > 7
Ammonia solution is basic

11. 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