What is pH?

What is pH?

Tells us the relative quantity of H+ ions (or
the acidity) in solution.

The greater the [H+] the lower the pH.
pH Values of Common Liquids
H+ and OH- in Pure Water
Self Ionization of Water: small quantity of water
molecules in a sample will self ionize.
Results in small, but equal [H+] and [OH-]
Concentration of these ions is 1 x 10-7 M (pH = 7)
Reversible reaction that is at equilibrium.
H 2O
H+
OH-
http://youtu.be/kW-Zk4zABzw
H+ + OH[H+] = [OH-]
When Acid is Added to Water

Increases the H+ concentration
 As
H+ increases, OH- decreases
[H+] > [OH-]
H+
OH-
When Base is Added to Water

Increases OH- concentration
 When
OH- increases, H+ decreases
[OH] > [H+]
H+
OH-
pH Scale
0
1
2
3 4 5
6
7 8 9 10 11 12 13 14
Basic
Acidic
Neutral
[H+]>[OH-]
[H+] = [OH-]
[OH-]>[H+]
Important Summary
Acidic:
[H+] > [OH-]
Basic:
[H+] < [OH-]
Neutral: [H+] = [OH-]
Pretty Hydrangeas

The color of hydrangea flowers depends
on the pH of the soil.
Calculating pH
pH = - log [H+]
 From the French pouvoir hydrogene
(“hydrogen power” or power of hydrogen)
[H+]
pH
1 x 10-5 M
5
1 x 10-9 M
9
1 x 10-11 M
11
11

If the [H+] is 0.000001 M, what is the pH?
pH = 6

If the [H+] is 0.01M, what is the pH?
pH = 2

If the pH is 10, what is the [H+]?
[H+] = 1 x 10-10 M

If the pH is 4, what is the [H+]?
[H+] = 1 x 10-4 M
Calculating pOH

What if you are given the concentration of
a base?

Ex: .000001M LiOH
You need to find the pOH!!
pOH
pOH = -log [OH-]
Ex:
.000001M LiOH = 1 x 10-6
pOH = 6
Ex:
.0001M NaOH = 1 x 10-4
pOH = 4
How can you find pH of a Base?

Easy!!!
pH + pOH = 14
Ex:
.000001M LiOH = 1 x 10-6
pOH = 6
pH = 8
Ex:
.0001M NaOH = 1 x 10-4
pOH = 4
pH = 10
Kw or Equilibrium Constant
for Water
If you know the concentration of one ion you
can find the other!
[H+] x [OH-] = 1 x 10-14

If [H+] is .001M what is the [OH-]?
[1 x 10-3] x [OH-] = 1 x 10-14
[OH-] = 1 x 10-11
Everything keeps adding up to 14!!
What’s up with that?
Let’s Try It
You have a .001 HCl solution.
Find: [H+], [OH-], pH, pOH
Let’s Try It
You have a .000000001M NaOH solution.
Find: [H+], [OH-], pH, pOH
Equation Summary
pH = -log [H+ ]
[H+ ] = 10-pH
pH + pOH = 14
pOH = -log [OH- ]
[OH- ] = 10-pOH
[H+ ] x [OH- ] = 1x10-14
Acidic solution:
Neutral solution:
Basic solution:
pH < 7 pOH > 7
pH = 7 pOH = 7
pH > 7 pOH < 7
Let’s Practice

A solution with a pH of 2.0 has a hydronium ion
concentration ten times greater than a solution
with a pH of
(1) 1
(2) 3
(3) 0.20
(4) 20

Which statement correctly describes a solution with
a pH of 9?
(1) It has a higher concentration of H3O+ than OH–
and causes litmus to turn blue.
(2) It has a higher concentration of OH–
than H3O+ and causes litmus to turn blue.
(3) It has a higher concentration of H3O+ than OH–
and causes methyl orange to turn yellow.
(4) It has a higher concentration of OH–
than H3O+ and causes methyl orange to turn red.

What is the pH of a solution that has a
hydronium ion concentration 100 times greater
than a solution with a pH of 4?
(1) 5
(2) 3
(3) 2
(4) 6

Solution A has a pH of 3 and solution Z has a
pH of 6.
How many times greater is the
hydronium ion concentration in solution A than
the hydronium ion concentration in solution Z?
(1) 100
(2) 3
(3) 2
(4) 1000

When the pH of a solution changes from a pH of 5
to a pH of 3, the hydronium ion concentration is
1). 0.01 of the original content
2). 0.1 of the original content
3). 10 times the original content
4). 100 times the original content

As the pH of a solution is changed from 3
to 6, the concentration of hydronium ions
(1) increases by a factor of 3
(2) increases by a factor of 1000
(3) decreases by a factor of 3
(4) decreases by a factor of 1000
Honors Questions
If the pH of a solution is 3.25 find the pOH,
[H+], and [OH-].
If .065 mole of KOH is placed in 20 liters of
water, what is the resulting pH?
Crash Course pH and pOH
 http://www.youtube.com/watch?v=LS67vS
10O5Y&safe=active

Acid Rain
(Not to be confused with
“Chocolate Rain”)
Acid Rain

Unpolluted rain has a
pH of 5.6

It is naturally acidic
due to the gases in the
air that dissolve in it.
Acid Rain

Rain with a pH below 5.6 is “acid rain“
pH of Rainwater
across United States in 2001
Sources of Acid Rain
Sources of Acid Rain

Power stations

Oil refineries

Coal with high S content

Car and truck emissions

Bacterial decomposition, and lightning
hitting N2

CO2 in the air forms carbonic acid
CO2 + H2O

H2CO3
This increases the [H+] of rain
H2CO3
H+ (aq) + HCO3-(aq)
 Reactions with oxygen in air form SO3
2SO2 + O2

2 SO3
Reactions with water in air form acids
SO3 + H2O
H2SO4 sulfuric acid
NO + H2O
HNO2 nitrous acid
HNO2 + H2O
HNO3 nitric acid
Effects of Acid Rain
Effects of Acid
Rain

Leaches heavy metals like Al from soil,
which kills fish

Fish kills in spring from runoff due to
accumulation of large amounts of acid in
snow

Prevents shellfish from building strong shells

Dissolves waxy coatings that protect leaves
from bacteria

Corrodes metals, textiles, paper and leather
Honors: pH and pOH game
 http://www.quia.com/rr/4051.html

Acid Rain Destroys National Monuments
http://news.discovery.com/videos/earth-acidrain-eating-washington-dc.html
Global Warming: CO2 and Ocean Acidity
http://planetgreen.discovery.com/videos/blue
-august-acid-in-the-water.html
Acid Rain Site produced by EPA
 http://www.epa.gov/acidrain/education/site
_kids/

EPA Website Detailing Acid Rain Causes
and Effects as well as Legislation
 http://www.epa.gov/acidrain/
