1. pH and pOH
Chem 332 – O’Dette

2. pH (Potential of Hydrogen)
Measures the degree of acidity or basicity of an aqueous solution
Corresponds to the hydrogen ion concentration, [H+]
Measured on a scale from 0-14
- pH < 7 → Acid [H+] > 10-7
- pH ≈ 7 → Neutral [H+] = 10-7
- pH > 7 → Base [H+] < 10-7

3. pH Scale Strong Acids Weak Acids Weak Bases Strong Bases Neutral 1 2 31 2 3 4 5 6 7 8 9 10 11 12 13 14
Strong Acids
Weak Acids
Weak Bases
Strong Bases
Neutral

4. pH of Different Solutions
Bases:
Bleach = 13
Soapy Water = 12
Ammonia Solution = 11
Milk of Magnesia = 10
Baking Soda = 9
Sea Water = 8
Acids:
Urine = 6
Black Coffee = 5
Tomato Juice = 4
Orange Juice = 3
Lemon Juice = 2
Gastric Acid =1
Neutral:
- Distilled Water = 7

5. Calculating pH pH = - log [H+] [H+] = 10-pH Based on a log scale
From [H+] → pH
From pH → [H+]
pH = - log [H+]
Memorize
[H+] = 10-pH

6. Step-Through Example Find pH of 0.2 M of Hydrochloric Acid
HCl(aq) → H+(aq) + Cl-(aq)
1 mole mole 1 mole
[0.2 M] → [0.2 M] [0.2 M]
pH = - log [H+] = - log [0.2M]
pH = 0.7

7. pOH (Potential of Hydroxide)
Similar to pH except pOH measures the hydroxide ion concentration, [OH-]

8. Calculating pOH pOH = - log [OH-] [OH-] = 10-pOH From [OH-] → pOH
From pOH → [OH-]
pOH = - log [OH-]
Memorize
[OH-] = 10-pOH

9. Relationship between pH and pOH
pH + pOH = 14
[H+] x [OH-] = 10-14
Memorize

10. Ex: Find the pH of 0.1 M of sulfurous acid, H2SO3
H2SO3 (aq) → 2 H+ (aq) + SO3-2 (aq)
1 mole moles mole
[0.1 M] → [0.2 M] [0.1 M]
pH = - log [H+] = - log [0.2M]
pH = 0.7

11. Ex: Find the pH of 0.005 M of Calcium Hydroxide, Ca(OH)2
Ca(OH)2 (aq) → Ca+ (aq) + 2OH- (aq)
1 mole mole moles
[0.005 M] → [0.005 M] + [0.01 M]
pOH = - log [OH-] = - log [0.01M]
pOH = 2
pH + pOH = 14
pH = 14 – pOH =
pH = 12