AP Chemistry Questions on Weak Acid-Strong Base Titrations

🧪 Quick Study Guide

• ⚖️ A titration is a process used to determine the concentration of an unknown solution (the *analyte*) by reacting it with a solution of known concentration (the *titrant*).
• 💧 In a weak acid-strong base titration, the weak acid only partially dissociates in water.
• 🔑 Key Concepts:
  • ⚛️ Initial pH: Calculated using the $K_a$ of the weak acid and an ICE table.
  • ➕ Before Equivalence Point: A buffer solution is formed, containing the weak acid and its conjugate base. The pH can be calculated using the Henderson-Hasselbalch equation: $pH = pK_a + log(\frac{[A^-]}{[HA]})$
  • 📍 At Equivalence Point: All the weak acid has reacted with the strong base to form its conjugate base. The pH is determined by the hydrolysis of the conjugate base. Use an ICE table to determine $[OH^-]$ and then calculate pOH and pH.
  • ➕ After Equivalence Point: The pH is determined by the excess strong base. Calculate the concentration of $OH^-$ and then calculate pOH and pH.
  • ⚗️ Half-Equivalence Point: The point at which half of the weak acid has been neutralized. At this point, $[HA] = [A^-]$, and $pH = pK_a$.
• 🧮 Calculations:
  • ➕ Use stoichiometry to determine the moles of reactants.
  • ➗ Determine the limiting reactant.
  • 📊 Use ICE tables to calculate equilibrium concentrations.
• 💡 Important Notes:
  • 📈 The pH at the equivalence point is *always* greater than 7 for a weak acid-strong base titration.
  • 📉 The titration curve will have a smaller pH change near the equivalence point compared to a strong acid-strong base titration.

Practice Quiz

1. What is the pH at the half-equivalence point in the titration of a weak acid with a strong base?
   1. pH = 7
   2. pH = $pK_a$
   3. pH = $pK_b$
   4. pH = 0
2. Which of the following is true at the equivalence point of a weak acid-strong base titration?
   1. pH = 7
   2. pH < 7
   3. pH > 7
   4. [Acid] = [Base]
3. What is the main buffer region in a weak acid-strong base titration curve?
   1. Before the addition of any base
   2. Near the equivalence point
   3. After the equivalence point
   4. Before the equivalence point
4. Which equation is used to calculate the pH in the buffer region of a weak acid-strong base titration?
   1. $pH = -log[H^+]$
   2. $pH = pK_a + log(\frac{[A^-]}{[HA]})$
   3. $pH = pK_a - log(\frac{[A^-]}{[HA]})$
   4. $pH = 14 - pOH$
5. During the titration of a weak acid with a strong base, what chemical species are present in significant concentrations *before* the equivalence point?
   1. Only the weak acid
   2. Only the conjugate base
   3. Both the weak acid and its conjugate base
   4. Neither the weak acid nor its conjugate base
6. If the $K_a$ of a weak acid is $1.8 \times 10^{-5}$, what is its $pK_a$?
   1. 1.8
   2. 5
   3. 4.74
   4. 9.26
7. What primarily determines the pH *after* the equivalence point in a weak acid-strong base titration?
   1. The concentration of the weak acid
   2. The concentration of the conjugate base
   3. The concentration of the excess strong base
   4. The $K_a$ of the weak acid