pH
Water
Water is in equilibrium with its ions
H2O(l)  H+(aq) + OH-(aq)
Kw = [H+][OH-]
 Kw = 1.0 x 10-14 at 25°C
 In neutral solutions [H+]=[OH-]= 1x10-7
 If [H+]>[OH-], solution is acidic
 If [H+]<[OH-], solution is basic

Example

Find the [H+] if
a) [OH-]= .010M
b) [OH-]= 2.0 x 10-9 M
pH
Very small numbers can be
conveniently expressed on a log scale
 pH = -log [H+]
 pH = 7, neutral
 pH < 7, acidic
 pH > 7, basic

Example

Calculate the pH of lemon juice with an
[H+]= 3.8 x 10-4M

Calculate the pH of window cleaner with
an [H+]= 5.3 x 10-9M

What is the [H+] of apple juice with a pH
of 3.76.
pOH

pOH = -log [OH-]


pOH = 7, neutral
pOH < 7, basic
pOH > 7, acidic

pH + pOH = 14.00

Indicators
Used to estimate pH
 Colored substance that exists in either
acid or base form
 Acid and base form are different colors
 By knowing the pH where it changes
color, we can determine its relative pH

Strong Acids and Bases
Strong acids: HCl, HBr, HI, HNO3,
HClO4, H2SO4 (only the 1st H)
 Strong bases: hydroxides of group I and
II, except Be





.10M HNO3  .10M H+
.75M H2SO4  .75M H+
.50M NaOH  .50M OH.75M Ca(OH)2  1.5M OH-
Example

What is the pH of a solution of
a) .010M HCl
b) .010M Ca(OH)2