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Buffer Solution Calculations Overview
Aug 22, 2024
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Buffer Solution Calculations using Henderson-Hasselbalch Equation
Introduction to Buffer Solutions
A buffer solution consists of a conjugate acid-base pair.
Example:
Acid: NH₄⁺ (ammonium)
Base: NH₃ (ammonia)
Henderson-Hasselbalch Equation
Formula:
pH = pKa + log([A⁻]/[HA])
Where:
[A⁻] = concentration of the base
[HA] = concentration of the acid
Step 1: Calculating pKa
Given:
Ka for NH₄⁺ = 5.6 x 10⁻¹⁰
Calculation of pKa:
pKa = -log(5.6 x 10⁻¹⁰) = 9.25
Step 2: Calculate Initial pH of Buffer Solution
Concentrations:
[A⁻] (NH₃) = 0.24 M
[HA] (NH₄⁺) = 0.20 M
Plug values into the equation:
pH = 9.25 + log(0.24/0.20)
log(0.24/0.20) = 0.08
Final pH = 9.25 + 0.08 = 9.33
Step 3: Adding Base to the Buffer Solution
Added:
0.005 moles of NaOH in 0.50 L
Calculate concentration of NaOH:
Concentration = moles/volume = 0.005/0.50 = 0.01 M
This is also the concentration of OH⁻ ions.
Reaction of NaOH with NH₄⁺
Reaction:
NH₄⁺ + OH⁻ → NH₃ + H₂O
Changes in concentrations:
NH₄⁺: 0.20 - 0.01 = 0.19 M
NH₃: 0.24 + 0.01 = 0.25 M
Step 4: New pH Calculation after Base Addition
Plug into Henderson-Hasselbalch Equation:
pH = 9.25 + log(0.25/0.19)
log(0.25/0.19) = 0.12
Final pH = 9.25 + 0.12 = 9.37
Conclusion:
Added base, pH increased from 9.33 to 9.37. Buffer resists drastic changes in pH.
Step 5: Adding Acid to the Buffer Solution
Added:
0.03 moles of HCl in 0.50 L
Calculate concentration of HCl:
Concentration = 0.03/0.50 = 0.06 M
This is also the concentration of H₃O⁺ ions.
Reaction of HCl with NH₃
Reaction:
NH₃ + H₃O ⁺ → NH₄⁺ + H₂O
Changes in concentrations:
NH₃: 0.24 - 0.06 = 0.18 M
NH₄⁺: 0.20 + 0.06 = 0.26 M
Step 6: New pH Calculation after Acid Addition
Plug into Henderson-Hasselbalch Equation:
pH = 9.25 + log(0.18/0.26)
log(0.18/0.26) = -0.16
Final pH = 9.25 - 0.16 = 9.09
Conclusion:
Added acid, pH decreased from 9.33 to 9.09. Buffer again resists drastic pH changes.
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