pH and pOH Chem 332

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

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

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

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

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

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

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

Relationship between pH and pOH pH + pOH = 14 [H+] x [OH-] = 1 x 10-14 Memorize

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

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