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. pH Scale 7 Acid Base 14 Acidic Neutral Basic [H+] pH 10-14 14 10-13 13
1 M NaOH
Ammonia
(household
cleaner)
Blood
Pure wate
Milk
Vinegar
Lemon juice
Stomach acid
1 M HCl
Acidic Neutral Basic
Acid
Base 7 14
pH Scale
“S.P.L. Sorensen ( ) introduced the pH scale to measure the concentration of hydrogen ions in solution. The more hydrogen ions, the stronger the acid. The amount of hydrogen ions in solution can affect the color of certain dyes found in nature. These dyes can be used as indicators to test for acids and alkalis. An indicator such as litmus (obtained from lichen) is red in acid. If base is slowly added, the litmus will turn blue when the acid has been neutralized, at about 6-7 on the pH scale. Other indicators will change color at different pH’s. A combination of indicators is used to make a universal indicator.”
- Eyewitness Science “Chemistry” , Dr. Ann Newmark, DK Publishing, Inc., 1993, pg 42
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 515

3. pH of Common Substances
Timberlake, Chemistry 7th Edition, page 335

4. 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
Strong
Base
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
Strong
Acid

5. 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

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

7. pH Calculations pH pOH [H3O+] [OH-] pH = -log[H3O+] [H3O+] = 10-pH
[H3O+] [OH-] = 1 x10-14
pOH
[OH-]
pOH = -log[OH-]
[OH-] = 10-pOH

8. pH = - log [H+] Given: pH = 4.6 determine the [hydronium ion]
choose proper equation
4.6 = - log [H+]
substitute pH value in equation
= log [H+]
multiply both sides by -1
2nd
log
= log [H+]
take antilog of both sides
[H+] = 2.51x10-5 mol/L
10x
antilog
Recall, [H+] = [H3O+]
You can check your answer by working backwards.
pH = - log [H+]
pH = - log [2.51x10-5 mol/L]
pH = 4.6

9. Acid Dissociation pH = ? pH = - log [H+] monoprotic HA(aq)
H1+(aq) + A1-(aq)
0.3 mol/L
0.3 mol/L
0.3 mol/L
pH = - log [3 mol/L]
e.g. HCl, HNO3
pH =
pH = - log [H+]
diprotic
H2A(aq)
2 H1+(aq) + A2-(aq)
0.3 mol/L
0.6 mol/L
0.3 mol/L
pH = - log [6 mol/L]
e.g. H2SO4
pH =
polyprotic
3 H1+(aq) + PO43-(aq)
Given: pH = 2.1
H3PO4(aq)
e.g. H3PO4
? mol/L
x mol/L
find [H3PO4]
assume 100%
dissociation

10. 3 H1+(aq) + PO43-(aq)
Given: pH = 2.1
H3PO4(aq)
X mol/L
mol/L
find [H3PO4]
assume 100%
dissociation
Step 1) Write the dissociation of phosphoric acid
Step 2) Calculate the [H+] concentration
pH = - log [H+]
[H+] = 10-pH
2.1 = - log [H+]
[H+] =
- 2.1 = log [H+]
[H+] = mol/L
2nd
log
- 2.1 = log [H+]
7.94 x10-3 mol/L
[H+] = x10-3 mol/L
Step 3) Calculate [H3PO4] concentration
Note: coefficients (1:3) for (H3PO4 : H+)
7.94 x10-3 M 3 =
mol/L H3PO4

11. How many grams of magnesium hydroxide are needed to add to 500 mL of H2O to yield a pH of 10.0?
Mg2+
Step 1) Write out the dissociation of magnesium hydroxide
OH1-
Mg(OH)2
Mg(OH)2 (aq)
Mg2+ (aq) +
2 OH1-(aq)
0.5 x10-4 mol/L
1 x10-4 mol/L
pH + pOH = 14
Step 2) Calculate the pOH
pOH = 14
pOH = 4.0
pOH = - log [OH1-]
Step 3) Calculate the [OH1-]
[OH1-] = 10-OH
[OH1-] = 1 x10-4 mol/L
Step 4) Solve for moles of Mg(OH)2
n = C V
n = 5 × 10-5mol/1 × 0.5 L
n = 5 x 10-5 mol Mg(OH)2
Step 5) Solve for grams of Mg(OH)2