Ka and Kb / Weak Acids and Bases
Related Examples and Practice Problems
Additional Worked Out Examples/ Practice
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Separation techniques: Selected and explaining limitation of appropriate separation
Relating Properties to Composition: Predicting classification based on descriptive properties
Topic Summary & Highlights
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Core Concept
Weak Acids: Acids that only partially dissociate into H+H^+H+ (or H3O+H_3O^+H3O+) and their conjugate base in solution.
Example: Acetic acid (CH_3COOH) dissociates as: CH_3COOH \rightleftharpoons H^+ + CH_3COO^-
Weak Bases: Bases that only partially accept protons (H^+) in solution.
Example: Ammonia (NH3NH_3NH3) reacts with water as: NH3+H2O⇌NH4++OH−NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-NH3+H2O⇌NH4++OH−
Practice Tips
Weak acids and bases do not fully dissociate; equilibrium must be considered.
$K_a$ and $K_b$ are measures of acid and base strength, respectively:
Larger $K_a$ or $K_b$: Stronger acid or base.
Smaller $K_a$ or $K_b$: Weaker acid or base.
Approximations simplify calculations, but always verify assumptions.
Use Ka ⋅ Kb = Kw to relate conjugate pairs.
Key Characteristics
| Property | Weak Acids | Weak Bases |
|---|---|---|
| Dissociation | Partial dissociation | Partial proton acceptance |
| Equilibrium | Establish equilibrium between dissociated and undissociated forms | Establish equilibrium between protonated and unprotonated forms |
| Ion Concentration | Produces fewer \( H^+ \) ions compared to strong acids | Produces fewer \( OH^- \) ions compared to strong bases |
| Conductivity | Poor conductor due to low ion concentration | Poor conductor due to low ion concentration |
| Strength Indicator | Measured by \( K_a \) | Measured by \( K_b \) |
Particle Level Diagram
Dissociation Constants
Acid Dissociation Constant (Ka)
Represents the strength of a weak acid: Ka=[H+][A−][HA]K_a = \frac{[H^+][\text{A}^-]}{[\text{HA}]}Ka=[HA][H+][A−]
[$H^+$]: Concentration of hydrogen ions.
[$\text{A}^-$]: Concentration of the conjugate base.
[$\text{HA}$]: Concentration of the undissociated acid.
Base Dissociation Constant (Kb)
Represents the strength of a weak base: Kb=[BH+][OH−][B]K_b = \frac{[\text{BH}^+][OH^-]}{[\text{B}]}Kb=[B][BH+][OH−]
[BH+][\text{BH}^+][BH+]: Concentration of the conjugate acid.
[OH−][OH^-][OH−]: Concentration of hydroxide ions.
[B][\text{B}][B]: Concentration of the unreacted base.
Relationship Between Ka, Kb, and Kw
For a conjugate acid-base pair: Ka⋅Kb=KwK_a \cdot K_b = K_wKa⋅Kb=Kw
Kw=1.0×10−14K_w = 1.0 \times 10^{-14}Kw=1.0×10−14 at 25°C.
Example:
For acetic acid (Ka=1.8×10−5K_a = 1.8 \times 10^{-5}Ka=1.8×10−5) and acetate ion: Kb=KwKa=1.0×10−141.8×10−5=5.6×10−10K_b = \frac{K_w}{K_a} = \frac{1.0 \times 10^{-14}}{1.8 \times 10^{-5}} = 5.6 \times 10^{-10}Kb=KaKw=1.8×10−51.0×10−14=5.6×10−10