1. pH Scale Soren Sorensen (1868 - 1939)
The pH scale was invented by the Danish chemist Soren Sorensen for a brewery to measure the acidity of beer.
Soren Sorensen
( )

2. Hold up! First we need to review!
Acids – Arrhenius Definition
Produce Hydronium ions (H3O+1) in water H O
water H O +1 H +1
Hydronium ion
By this definition, if an acid is to give a H+1 to water, then all acids will have hydrogen as the cation (first element written).
Bases – Arrhenius Definition
Bases produce the hydroxide ion in water H O -1
Hydroxide ion

3. Ok, here we go… the pH Scale
It is a scale to measure the acidity of a sample. It measures the concentration of hydronium (H3O+1)ions in the solution. 1 14
Highly acidic
Very basic (not acidic)
neutral 7

4. pH of Common Substances
vinegar
2.8
water (pure)
7.0
soil
5.5
gastric
juice
1.6
carbonated
beverage
3.0
drinking water
7.2
bread
5.5
1.0 M
NaOH
(lye)
14.0
orange
3.5
potato
5.8
blood
7.4
1.0 M
HCl
milk of
magnesia
10.5
apple juice
3.8
urine
6.0
detergents
bile
8.0
lemon
juice
2.2
tomato
4.2
milk
6.4
ammonia
11.0
seawater
8.5
coffee
5.0
bleach
12.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
acidic
neutral
basic
[H+] = [OH-]
Timberlake, Chemistry 7th Edition, page 335

5. Each step on pH scale represents a factor of 10.
How does the scale work?
pH scale
: measures acidity/basicity
Søren Sorensen
( )
ACID
BASE
10x
10x
10x
NEUTRAL
Each step on pH scale represents a factor of 10.
pH is defined as the negative base-10 logarithm of the hydrogen ion concentration
pH = – log [H+] or [H+] = 10-pH
Hydrogen ion concentration in pure water is 1 x 10-7 M at
25ºC; the pH of pure water is – log [1.0 x 10-7] = 7.00.
pH decreases with increasing [H+] — adding an acid to pure water increases the hydrogen ion concentration and decreases the hydroxide ion concentration.
Adding a base to pure water increases the hydroxide ion concentration and decreases the hydrogen ion concentration—pH increases with decreasing [H+].
pH scale runs from pH = 0 (corresponding to 1 M H+) to pH 14 (corresponding to 1 M OH–).
Relationships between acidity, basicity, and pH:
If pH = 7.0, the solution is neutral.
If pH < 7.0, the solution is acidic.
If pH > 7.0, the solution is basic.
A change of 1,0 in the pH of a solution corresponds to a tenfold change in the hydrogen ion concentration because the pH scale is logarithmic.
pH 5 vs. pH (10X more acidic)
pH 3 vs. pH (100X different)
pH 8 vs. pH (100,000X different)

6. Richter Scale - Earthquakes6 5 7
Area Radius Richter Scale Diameter pH [H+]
mm M M M M M M M . 4 3 2 1

7. radius = 112.8 cm radius = 35.7 cm radius = 11.3 cm radius = 3.6 cm
pH = 2
pH = 3
pH = 1
Area Radius Richter Scale Diameter pH
mm 1 .
pH =

8. The pH Scale
Acid
Base
pH = 7
Acidic
Basic
[H+]
[OH-]
[H+] =
Neutral

9. pH of Common Substance 14 1 x 10-14 1 x 10-0 0 13 1 x 10-13 1 x 10-1 1
pH [H1+] [OH1-] pOH
14 1 x x
13 1 x x
12 1 x x
11 1 x x
10 1 x x
9 1 x x
8 1 x x
6 1 x x
5 1 x x
4 1 x x
3 1 x x
2 1 x x
1 1 x x
0 1 x x
NaOH, 0.1 M
Household bleach
Household ammonia
Lime water
Milk of magnesia
Borax
Baking soda
Egg white, seawater
Human blood, tears
Milk
Saliva
Rain
Black coffee
Banana
Tomatoes
Wine
Cola, vinegar
Lemon juice
Gastric juice
More basic
7 1 x x
More acidic

10. pH
pH = -log [H1+]
Kelter, Carr, Scott, Chemistry A World of Choices 1999, page 285

11. Acid – Base Concentrations
10-1
pH = 3
pH = 11
H3O+
OH-
pH = 7
10-7
concentration (moles/L)
H3O+
OH-
OH-
H3O+
10-14
[H3O+] > [OH-]
[H3O+] = [OH-]
[H3O+] < [OH-]
acidic
solution
neutral
solution
basic
solution
Timberlake, Chemistry 7th Edition, page 332

12. pH Calculations pH pOH [H3O+] [OH-] pH + pOH = 14 pH = -log[H3O+]
[H3O+] = 10-pH
[H3O+] [OH-] = 1 x10-14
The pH scale is a concise way of describing the H3O+ concentration and the acidity or basicity of a solution
• pH and H+ concentration are related as follows:
pH = –log10[H+] or [H+] = 10–pH
• pH of a neutral solution ([H3O+] = 1.00 x 10–7 M) is 7.00
• pH of an acidic solution is < 7, corresponding to [H3O+] > 1.00 x 10–7
• pH of a basic solution is > 7, corresponding to [H3O+] < 1.00 x 10–7
• The pH scale is logarithmic, so a pH difference of 1 between two solutions corresponds to a difference of a factor of 10 in their hydronium ion concentrations
There is an analogous pOH scale to describe the hydroxide ion concentration of a solution; pOH and [OH–] are related as follows:
pH = –log10[OH–] or [OH–] = 10–pOH
• A neutral solution has [OH–] = 1.00 x 10–7, so the pOH of a neutral solution is 7.00
• The sum of the pH and the pOH for a neutral solution at 25ºC is = 14.00
pKw = –log Kw = –log([H3O+] [OH–]) =
(–log[H3O+]) + (–log[OH–]) = pH + pOH
• At any temperature, pH + pOH = pKw, and at 25ºC, where Kw = 1.01 x 10–14, pH + pOH = 14.00; pH of any neutral solution is just half the value of pKw at that temperature
[OH-]
pOH = -log[OH-]
[OH-] = 10-pOH

13. pH + pOH = 14 pH
pOH
pH + pOH = 14
This is true because 1 to 14 is the whole range of the scale. Therefore, what ever part is not hydronium ions is composed of hydroxide ions.

14. Auto-ionization of Water
[H3O+]
Water will split into ions
2 H2O  H3O+1 + OH-1
Water will do this to make sure that they come to equilibrium and this is true:
[H3O+1] × [OH-1] = 1 × 10-14
Therefore, if you know the concentration of one you always know the concentration of the other! How handy!
[H3O+] [OH-] = 1 x10-14
[OH-]

15. Example Problems !!
Example:
Find the pH of an acid if the pOH is 12

16. Example Problems !! Example:
Find the pH if the concentration of [H3O+1] is 0.25 M
Is the pH acidic or basic?

17. Example Problems !!
Example:
Find the [H3O+1] if the pH is 2.7

18. Example Problems !! Example: Find the [OH-1] if the pH is10.7
***Keep in mind there is more than one way to go about this***