3.1.12.3 Weak Acids and Bases

• What is the key difference between strong and weak acids/bases in terms of dissociation?

  Strong acids/bases dissociate completely
• Weak acids exist in equilibrium with their undissociated form.


  True
• Match the terms with their meanings:

  HA ↔️ Undissociated weak acid

  H⁺ ↔️ Hydrogen ion

  A⁻ ↔️ Conjugate base
• The Ka value is specific to each weak acid.

  True
• Weak acids exist in equilibrium
• HA in the dissociation reaction represents the undissociated weak acid


  True
• A higher Ka value indicates a stronger weak acid

  True
• The formula for Ka is $K_{a} =$$\frac{[H^ + ][A^ - ]}{[HA]}$


  True
• Weak acids have a moderately low pH, while weak bases have a moderately high pH.
• The acid dissociation constant (Ka) is specific to each weak
• A higher Ka value indicates a stronger
• What is the Ka value of acetic acid?

  $1.8 x 10^{ - 5}$
• For weak acids, the ICE method is used because they only dissociate partially
• The formula to calculate pH is $pH =$$- \log[H⁺]$.


  True
• Use the $K_{a}$ expression and the equilibrium concentrations to solve for x.
• At equilibrium, the concentration of $CH_{3}COOH$ is 0.1 - x.
• The pK_a is defined as pK_{a} = - \log K_{a}</latex>.


  True
• Steps in the dissociation of a weak base

  1️⃣ $B(aq) +$$H₂O(l) ⇌ HB⁺(aq) +$$OH⁻(aq)$

  2️⃣ Quantify the extent of dissociation using $K_{b}$

  3️⃣ Calculate $[OH⁻]$ from $K_{b}$

  4️⃣ Determine the $pH$
• What is the primary difference between strong and weak bases regarding their dissociation in water?

  Complete vs. partial
• What happens to weak acids and weak bases when dissolved in water?

  Partially dissociate into ions
• What is the typical pH range for weak acids?

  Moderately low pH
• What does HA represent in the dissociation reaction of a weak acid?

  Undissociated weak acid
• What does Ka measure for a weak acid?

  Extent of dissociation
• What is the Ka value of acetic acid?

  1.8 × 10^{ - 5}</latex>
• The pH of a 0.1 M acetic acid solution is approximately 2.87.

  True
• When pH equals pKa, the concentrations of acid and conjugate base are equal.


  True
• A lower pK_a value indicates a stronger acid.


  True
• The dissociation of a weak base in water involves an equilibrium between the base, hydroxide ions, and its conjugate acid.
• Match the feature with the type of base:

  Complete dissociation in water ↔️ Strong Bases

  Partial dissociation in equilibrium ↔️ Weak Bases
• A higher $K_{b}$ value indicates a greater concentration of hydroxide ions at equilibrium.


  True
• Steps to calculate the pH of a weak base solution:

  1️⃣ Write the balanced dissociation reaction

  2️⃣ Set up an ICE table

  3️⃣ Determine change in concentrations

  4️⃣ Calculate equilibrium concentrations

  5️⃣ Use the $K_{b}$ expression

  6️⃣ Approximate if $x < < [B]_{0}$

  7️⃣ Calculate $[OH⁻]$, pOH, and finally pH
• What is the pH of a 0.2 M ammonia ($NH₃$) solution with $K_{b} =$$1.8 \times 10^{ - 5}$?


  $pH ≈ 11.28$
• Assuming x << 0.2, the term (0.2 - x) can be approximated to 0.2 for simplification


  True
• What is the approximate pH of the solution in the example calculation?

  11.28</latex>
• $K_{b} =$$\frac{[HB⁺][OH⁻]}{[B]} =$$\frac{x²}{[B]_{0} - x}$ is the equilibrium expression for a weak base
• Match the feature with the type of acid or base:

  Complete dissociation ↔️ Strong Acids/Bases

  Equilibrium exists ↔️ Weak Acids/Bases

  Low pH ↔️ Strong Acids

  Moderate pH ↔️ Weak Acids
• What does HA represent in the dissociation of a weak acid?

  Undissociated weak acid
• A lower $K_{a}$ value indicates a weaker acid


  True
• What does a higher $K_{a}$ value indicate for a weak acid?


  Stronger weak acid
• Which acid is stronger: acetic acid or formic acid?

  Formic acid