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 and bases ionize completely in solution

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