What is
pH?
Ion Product Constant for Water
 H2O(l)
 Keq
 H+(aq) + OH-(aq)
= Kw = [H+] x [OH-]
 The
ion product constant for water (Kw) is the
equilibrium constant (Keq) expression for the
self-ionization of water.
 Kw
= 1 x 10-14 at 298K.

H2O(l)  H+(aq) + OH-(aq)

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 10-5M. 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 10-6M. 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
ranges from 0-14
pH <7 is acidic
pH = 7 is neutral
pH > 7 is basic
pH= -log[H+]
***Always use pH
to identify if a
solution is acidic,
neutral or basic,
NOT pOH
What is pH?


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.

***There is a shortcut
to this, look at the
eponent for the given
concentration, and that
is your pH or pOH
depending on which
concentration is given
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-]
pH = -log[H+]
1 x 10-14 = [H+] x [9.6 x 10-4] pH = -log[1.0 x 10-11]
9.6 x 10-4
9.6 x 10-4
pH = 11
1.0 x 10 -11 = [H+]
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 = 3 (from the exponent)
pH + pOH = 14
pH + 3 = 14
pH > 7 is basic
pH = 11
11 > 7
Ammonia 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 = 8
pH + pOH = 14
8+ pOH = 14
pOH = 6
pH > 7 is basic
8>7
solution is basic