Acids and Bases IIWater Dissociation

and pH

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

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

Water Dissociation (cont.)

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-]

Acid/Base Addition

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

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)

Increase [H3O+], decrease pH value, decrease [OH-]

Decrease [H3O+], increase pH value, increase [OH-]

pH scale (cont.)

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

What is the pH of Pepsi Cola if the [H3O+] in the

solution is 0.0035M?

Example 1

Strong acids completely dissociate in solution

SOOO solution’s [H3O+] ~ [H3O+] in acid All [H3O+] ions resulting from acid

pH and strong acids

Find the pH of a 2.5M HCl solution.

Example 2

Find the pH of a 0.05M H2SO4 solution

Example 3

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

Why is the [H3O+] from water dissociation not a

factor?

Acid/Base Introduction Worksheet

pH calculation Problems

Homework