Lecture Notes: Acid-Base Titrations

Introduction

• Focus on acid-base titrations.
• Discussion on titration curves and calculating pH at various points in the process.

Problem 1: Determining Concentration of H2SO4

• Given: 28.9 mL of H2SO4 titrated with 38.4 mL of 0.25 M NaOH.
• Objective: Find concentration of H2SO4.
• Methods:
  • Stoichiometry:
    • Balance chemical equation: H2SO4 + 2 NaOH → Na2SO4 + 2 H2O.
    • Convert volumes to liters.
    • Use molarity to find moles of NaOH.
    • Convert moles of NaOH to moles of H2SO4 using molar ratio.
    • Determine concentration by dividing moles of H2SO4 by original volume in liters.
    • Result: [H2SO4] = 0.1661 M.
  • Equation Method (M1V1 = M2V2):
    • Modify equation considering the 1:2 molar ratio.
    • Balance through number of protons and hydroxide ions.
    • Result: Same as stoichiometry method: [H2SO4] = 0.1661 M.

Problem 2: Volume of NaOH for Equivalence Point

• Given: 23.6 mL of 0.46 M monoprotic acid titrated with 0.19 M NaOH.
• Objective: Find volume of NaOH at equivalence point.
• Method:
  • Use M1V1 = M2V2, considering 1:1 molar ratio for monoprotic acid.
  • Result: Volume needed is 57.14 mL.

Titration Curves

Strong Acid-Strong Base Titration

• Initial low pH with strong acid, rises dramatically, equivalence point at pH 7.
• Inverse for strong base-strong acid: starts high, decreases, same equivalence point.

Weak Acid-Strong Base Titration

• Starts at higher pH than strong acid titration.
• Equivalence point is above pH 7.
• At half equivalence point, pH = pKa.

Weak Base-Strong Acid Titration

• Starts relatively high, but not as high as strong base.
• Equivalence point pH is less than 7.
• At half equivalence, concentrations of acid and base are equal.

Important Equations and Concepts

• Henderson-Hasselbalch Equation: pH = pKa + log([Base]/[Acid])
• Buffer Region: Resist changes in pH, ph = pKa at half equivalence point.
• Calculating pH at Equivalence Point:
  • Strong acid-strong base: pH = 7.
  • Weak acid-strong base: pH > 7.
  • Weak base-strong acid: pH < 7.

Example Problem: Strong Acid-Strong Base Titration

• Given: 50 mL of 1 M HCl titrated with 0.50 M NaOH.
• Objective: Calculate pH at various stages.
• Stages:
  • Calculate equivalence volume: 100 mL.
  • Initial pH of HCl solution: 0 (from concentration of H+).
  • pH after adding 30 mL NaOH: Use moles of HCl, NaOH.
  • pH at equivalence: 7.
  • pH after 125 mL NaOH: Calculate excess OH-, find pOH, then pH = 12.85.

Next Topic

• Discussion will continue with weak acid and strong base titration examples.

These notes provide a comprehensive overview of acid-base titrations, focusing on the calculations and concepts relevant to determining pH and titration curves.