Acid Dissociation Constant (Ka) Examples: Acetic Acid, Formic Acid, and More

📚 Quick Study Guide

• 🧪 The acid dissociation constant, $K_a$, quantifies the strength of an acid in solution. It represents the equilibrium constant for the dissociation of an acid (HA) into its conjugate base (A-) and a proton (H+).
• ➗ The general equilibrium reaction for acid dissociation is: $HA \rightleftharpoons H^+ + A^-$.
• 🔢 The expression for $K_a$ is: $K_a = \frac{[H^+][A^-]}{[HA]}$. Larger $K_a$ values indicate stronger acids.
• 🌡️ $pK_a$ is often used, where $pK_a = -log_{10}(K_a)$. A lower $pK_a$ signifies a stronger acid.
• 📝 Acetic acid ($CH_3COOH$) has a $K_a$ of approximately $1.8 \times 10^{-5}$ (or $pK_a$ ≈ 4.76).
• 🐜 Formic acid ($HCOOH$) has a $K_a$ of approximately $1.8 \times 10^{-4}$ (or $pK_a$ ≈ 3.75), making it a stronger acid than acetic acid.
• 💡 The strength of an acid is influenced by factors such as electronegativity and resonance stabilization of the conjugate base.

🧪 Practice Quiz

1. Which of the following best describes the acid dissociation constant, $K_a$?
1. A) The rate constant for acid dissociation.
2. B) The equilibrium constant for acid dissociation.
3. C) The concentration of acid at equilibrium.
4. D) The pH of the acid solution.
2. What does a larger $K_a$ value indicate?
1. A) A weaker acid.
2. B) A stronger acid.
3. C) A neutral solution.
4. D) An alkaline solution.
3. What is the approximate $K_a$ of acetic acid ($CH_3COOH$)?
1. A) $1.8 \times 10^{-3}$
2. B) $1.8 \times 10^{-4}$
3. C) $1.8 \times 10^{-5}$
4. D) $1.8 \times 10^{-6}$
4. What is the relationship between $K_a$ and $pK_a$?
1. A) $pK_a = log_{10}(K_a)$
2. B) $pK_a = -log_{10}(K_a)$
3. C) $K_a = -log_{10}(pK_a)$
4. D) $K_a = \frac{1}{log_{10}(pK_a)}$
5. Which acid is stronger: acetic acid ($K_a \approx 1.8 \times 10^{-5}$) or formic acid ($K_a \approx 1.8 \times 10^{-4}$)?
1. A) Acetic acid
2. B) Formic acid
3. C) They are of equal strength.
4. D) Cannot be determined.
6. What does a lower $pK_a$ value signify?
1. A) A weaker acid.
2. B) A stronger acid.
3. C) A neutral solution.
4. D) An alkaline solution.
7. Which of the following factors can influence the strength of an acid?
1. A) Electronegativity.
2. B) Resonance stabilization of the conjugate base.
3. C) Temperature.
4. D) Both A and B.