Water Dissociation

 Water acts as both acid and base
 Dissociates into H3O+ and OH- ions
 2H2O(l) 
H3O+(aq) + OH-(aq)
 Rewritten,
 H2O(l) + H2O(l)  H3O+(aq) + OH-(aq)
Water Dissociation
(cont.)

 Rate of reaction reaches equilibrium and allows
[H3O+] and [OH-] to be determined
 2H2O(l) 
H3O+(aq) + OH-(aq)
 Kw = [H3O+] [OH-]
 Kw = water dissociation constant = 1x10-14for pure
water
 [H3O+] = 1x10-7 M in pure water
 [OH-] = 1x10-7 M in pure water
Acid/Base Addition

 Kw = [H3O+] [OH-] = 1 x 10-14 regardless of where
ions come
 More acid, increase [H3O+] /decrease [OH-]
 2 sources for [H3O+] , so less water dissociation
 More base, decrease [H3O+] /increase [OH-]
pH

 Measure of the concentration of [H3O+] ions in an
acidic/basic solution
 Uses logarithmic scale
 pH = -log[H+] or –log[H3O+]
 [H3O+] = 10-pH
 pOH = -log[OH-]
 pH + pOH = 14
pH Scale

Logarithmic scale.
Measures the concentration of hydrogen ions
[H+] in a solution.
Range from 0-14.
NEUTRAL, pH=7. (pure water)
BASE, pH > 7. (ocean water, milk of
magnesia, baking sodea)
ACID, pH < 7. (stomach acid/HCl, vinegar,
soft drinks)
pH scale (cont.)

 Increase [H3O+], decrease pH value, decrease [OH-]
 Decrease [H3O+], increase pH value, increase [OH-]
pH Scale

How do we measure the
pH of a solution?

Acid-base indicators
Weak acids/bases
(ex. litmus paper)
pH meter
Equations

pH = -log[H+]
pOH = -log[OH-]
pH + pOH = 14
[H+][OH-] = 1x10 -14 M2
Example 1

 What is the pH of Pepsi Cola if the [H3O+] in the solution
is 0.0035M?
pH and strong acids

 Strong acids completely dissociate in solution
 SOOO solution’s [H3O+] ~ [H3O+] in acid
 All [H3O+] ions resulting from acid
Example 2

 Find the pH of a 2.5M HCl solution.
Example 3

 Find the pH of a 0.05M H2SO4 solution
+
O]
Why is the [H3
from water
dissociation not a factor?

1) [H3O+] = 1x10-7 M in water
 not a big number
2) LeChatlier’s Principle
 As more H3O+ ions dissociate from water, reaction shifts to
LEFT to compensate
 If more H3O+ ions added to solution, water reforms to try
and compensate SOOO decrease in [H3O+] from water
dissociation
Homework

 Acid/Base Introduction Worksheet
 pH calculation Problems