Lecture Notes: Understanding Titrations

Introduction

• Presenter: Robert Smith
• Institution: New River Community College's Academic Assistance
• Topic: Titrations
• Goal: Gain a better understanding of how to solve titration problems by working through examples.

Problem 1: Calculating Molarity of KOH

• Given:
  • Volume of KOH solution: 32.84 mL
  • Mass of HCl: 0.4586 grams
• Objective: Find the molarity of KOH

Steps to Solve

1. Convert Volume: Convert 32.84 mL to liters.
   • Formula: (32.84 \text{ mL} \times \frac{1 \text{ L}}{1000 \text{ mL}} = 0.03284 \text{ L})
2. Convert Mass to Moles:
   • Find molar mass of HCl: (35.45 + 1.008 = 36.46 \text{ g/mol})
   • Calculate moles: (0.4586 \text{ g} \times \frac{1 \text{ mol}}{36.46 \text{ g}} = 1.258 \times 10^{-2} \text{ mol})
3. Calculate Molarity:
   • Formula: (\text{Molarity} = \frac{\text{moles}}{\text{liters}})
   • Calculation: (\frac{1.258 \times 10^{-2} \text{ mol}}{0.03284 \text{ L}} = 0.3831 \text{ M})

• Conclusion: Molarity of KOH is 0.3831 M.

Problem 2: Volume of KOH Needed for Neutralization

• Given:
  • Molarity of KOH: 0.160 M
  • Volume of H3PO4: 30.0 mL
  • Molarity of H3PO4: 0.425 M
• Objective: Find the volume of KOH needed to neutralize H3PO4.

Steps to Solve

1. Convert Volume: 30.0 mL to liters
   • (30.0 \text{ mL} = 0.0300 \text{ L})
2. Account for Equivalency:
   • H3PO4 produces 3 H+ ions vs KOH producing 1 OH- ion (3:1 ratio)
3. Find Moles of H3PO4:
   • (0.425 \text{ M} \times 0.0300 \text{ L} = 0.0128 \text{ mol})
4. Calculate Moles of KOH Needed:
   • (3 \times 0.0128 \text{ mol} = 0.0384 \text{ mol})
5. Calculate Volume of KOH:
   • Volume = (\frac{\text{moles}}{\text{molarity}})
   • (\frac{0.0384 \text{ mol}}{0.160 \text{ M}} = 0.240 \text{ L})
6. Convert to Milliliters:
   • (0.240 \text{ L} \times 1000 \frac{\text{mL}}{\text{L}} = 240 \text{ mL})

• Conclusion: 240 mL of 0.160 M KOH is needed to neutralize the H3PO4 solution.

Key Takeaways

• Always convert units appropriately when dealing with molarity problems.
• Pay attention to the stoichiometry and equivalency of reactions.
• Double-check the final units to ensure they match what the problem asks for.