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Understanding Acid-Base Titrations

May 9, 2025

Acid-Base Titration Lecture Notes

Introduction to Acid-Base Titrations

  • Focus on titration curves and calculating pH at various points.
  • Discussed examples and problems related to acid-base titration.

Problem 1: Calculate Concentration of H₂SO₄

  • Given: 28.9 mL H₂SO₄ titrated with 38.4 mL of 0.25 M NaOH.
  • Objective: Find concentration of H₂SO₄.
  • Methods:
    1. Stoichiometry Approach:
      • Balanced equation: H₂SO₄ + 2 NaOH → Na₂SO₄ + 2 H₂O.
      • Use molarity and volume to find moles of NaOH.
      • Use mole ratio (1:2) to find moles of H₂SO₄.
      • Convert volume of H₂SO₄ to liters and calculate concentration.
    2. Modified M₁V₁ = M₂V₂ Equation:
      • Incorporate 1:2 molar ratio.
      • Calculate using given volumes and concentrations.
  • Result: Concentration of H₂SO₄ = 0.1661 M.

Problem 2: Equivalence Point Volume Calculation

  • Given: 23.6 mL of 0.46 M monoprotic acid titrated with 0.19 M NaOH.
  • Objective: Find volume of NaOH to reach equivalence point.
  • Method:
    • Use M₁V₁ = M₂V₂ for monoprotic acid.
    • Equivalence point is where moles of H⁺ equals moles of OH⁻.
  • Result: Volume of NaOH = 57.14 mL.

Titration Curves

Strong Acid-Strong Base Titration

  • pH starts low, increases sharply, then levels off.
  • Equivalence Point pH: 7.
  • Curve reflects addition of a strong base.

Weak Acid-Strong Base Titration

  • Starts at a higher pH than strong acid.
  • Equivalence Point pH: >7.
    • Depends on the acid's Ka.
    • At equivalence, weak base remains.
  • Half Equivalence Point:
    • pH = pKa.

Weak Base-Strong Acid Titration

  • Starts at relatively high pH.
  • Equivalence Point pH: <7.
    • Weak acid remains at equivalence.
  • Half Equivalence Point: Related to Henderson-Hasselbalch equation.

Buffer Region and Equivalence Point

  • Buffer solution: Equal amounts of acid and conjugate base.
  • Equivalence Point: Steep slope in pH changes.
  • Buffering Capacity: Ability to resist pH change.

Practical Problems

Example: Strong Acid-Strong Base Titration

  • 50 mL of 1 M HCl titrated with 0.50 M NaOH:
    • Equivalence Point Volume: 100 mL of NaOH.
    • Initial pH of HCl: 0 (from 1 M H⁺ concentration).
    • pH after Adding 30 mL NaOH:
      • Use ICF table for moles calculation.
      • Resulting pH = 0.359 (from excess H⁺).
    • Equivalence Point pH: 7.
    • pH after 125 mL NaOH:
      • Calculate excess OH⁻ concentration.
      • Resulting pH = 12.85.

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