Titration of a Weak Base with a Strong Acid

Overview

• Titration curve analysis for weak base titrated with strong acid
• Calculated pH at different points of titration

Key Points

Initial pH Calculation

• Calculate pH before adding any acid
• Initial conditions:
  • Weak base concentration: 0.1 M

pH Calculation After Adding 20ml Acid

• Previous calculation for point b in titration
• Acid added: 20ml of 0.1 M HCl

pH Calculation After Adding 40ml Acid

• Calculate pH after adding 40ml of 0.1 M HCl

Moles of Acid Added

• Concentration of HCl: 0.1 M
• Volume added: 40ml (0.04 L)
• Moles of HCl = 0.1 * 0.04 = 0.004 moles
• H⁺ ions = 0.004 moles*

Moles of Base (Ammonia) Present

• Initial concentration: 0.1 M
• Volume: 40ml (0.04 L)
• Moles of NH₃ = 0.1 * 0.04 = 0.004 moles*

Neutralization Reaction

1. NH₃ + H₃O⁺ → NH₄⁺ + H₂O
2. NH₃ + HCl → NH₄⁺ + Cl⁻

Equivalence Point

• Moles of acid = moles of base (0.004 moles each)
• Equivalence point reached
• Resulting NH₄⁺ concentration: 0.05 M

Reaction with Water

• NH₄⁺ acts as a weak acid
• NH₄⁺ + H₂O → NH₃ + H₃O⁺

Equilibrium Concentrations

• Initial NH₄⁺: 0.05 M
• Assume NH₃ and H₃O⁺ start at 0
• Change: -x for NH₄⁺, +x for NH₃ and H₃O⁺

Equilibrium Expression

• Ka = (x)(x) / (0.05 - x)
• Ka for NH₄⁺: 5.6 x 10⁻¹⁰
• Solve for x, concentration of H₃O⁺: x = 5.3 x 10⁻⁶

pH at Equivalence Point

• pH = -log(5.3 x 10⁻⁶) = 5.28

pH Calculation After Adding 60ml Acid

• Added 60ml of 0.1 M HCl
• Total moles of HCl: 0.006
• Reacted with 0.004 moles NH₃, excess HCl: 0.002 moles
• Total volume: 40ml (base) + 60ml (acid) = 100ml (0.1 L)

Concentration of Hydronium Ions

• [H₃O⁺] = 0.002 / 0.1 = 0.0200 M

Final pH Calculation

• pH = -log(0.0200) = 1.70

Conclusion

• At 60ml, the pH is 1.7, indicating that the solution is acidic and past the equivalence point.