pH Scale

H2CO3 + H2O ⇄ H3O+(aq) + HCO3−(aq) pH Scale The pH scale is used to rank the strength of acids and bases. Strong acids and bases completely dissociate (separate into ions) in water. HCl + H2O  H+(aq) + Cl−(aq) Weak acids and bases only partially dissociate in water. H2CO3 + H2O ⇄ H3O+(aq) + HCO3−(aq)

pH Scale List of Strong Acids List of Strong Bases HCl LiOH HBr NaOH HI KOH H2SO4 Mg(OH)2 HClO4 Ca(OH)2

brackets signify concentration pH Scale pH values are a measure of the concentration of hydrogen ions (H+) present in a solution. pH = −log [H+] brackets signify concentration Examples If [H+] = 1 x 10−4, then the pH = 4 If [H+] = 1 x 10−8, then the pH = 8

pH Scale In pure water, a very small percentage of water molecules naturally separate into hydrogen (H+) and hydroxide (OH−) ions. In pure water: [H+] = [OH−] = 1 x 10−7 M pH = 7

pH Scale Acids will increase the concentration of H+ (lowering the pH), and bases will increase the concentration of OH− (raising the pH).

pH Scale

pH Scale Example 1 What is the pH of a solution that has a hydrogen ion concentration equal to 0.003 M? pH = −log [H+] pH = −log (0.003) pH = 2.52

pH Scale For strong acids, the concentration of hydrogen ions (H+) is approximately equal to the concentration of the acid itself. Example 2 What is the pH of a 0.5 M solution of HCl? [H+] = [HCl] [H+] = 0.5 pH = −log [H+] pH = −log (0.5) pH = 0.3

pH Scale For weak acids, the concentration of hydrogen ions (H+) depends on the acid involved. The acid dissociation constant, Ka, is a value that relates the acid, conjugate base, and the hydrogen ion concentrations. We will only deal with buffer solutions containing equal concentrations of the acid and the conjugate base, so Ka = [H+].

pH Scale Example 3 For hydrofluoric acid, Ka = 6.8 x 10−4. If the concentrations of HF and F − are both 0.2 M, what is the pH of this solution? At equal concentrations, [H+] = Ka [H+] = 6.8 x 10−4 pH = −log [H+] pH = −log (6.8 x 10−4) pH = 3.17

pH Scale Buffer solutions contain a weak acid and its conjugate base, and act to keep the pH relatively constant even when small amounts of a strong acid or base are added. Example: Buffer of Ethanoic Acid and Sodium Ethanoate Adding H+ produces additional ethanoic acid (CH3COOH). Adding OH− produces additional ethanoate ions (CH3COO−). The pH changes very little.

pH Scale 1. Which of the following is a strong acid? a) HCOOH b) H2CO3 c) H3PO4 d) HCl

pH Scale 2. How is a strong acid different from a weak acid? a) Strong acids always have a lower pH. b) Strong acids completely dissociate in water. c) Strong acids always have a higher pH. d) Strong acids partially dissociate in water.

pH Scale 3. What is the pH of a solution containing an H+ concentration of 3.5 x 10−10? a) 4.5 b) 7.5 c) 9.5 d) 12.5

pH Scale 4. A buffer solution is made with 0.8 M acetic acid (Ka = 1.8 x 10−5) and 0.8 M sodium acetate. What is the pH of this solution? a) 0.1 b) 2.4 c) 4.7 d) 7.9

pH Scale 5. The addition of 10 mL of 0.1 M NaOH increases the pH of one liter of water from 7.0 to 11.0. The addition of 10 mL of 0.1 M NaOH added to the same volume of a buffer solution will… a) cause a smaller increase in pH. b) cause a larger increase in pH. c) cause the same increase in pH. d) It is impossible to tell