Multiple Choice
A buffer solution has 0.625 M HCN and 0.575 M CN⁻. If 0.030 mol of HCl is added to 400. mL of the buffer solution, what is the pH after the addition? The pKa of hydrocyanic acid is 9.23.
18. Aqueous Equilibrium
Henderson-Hasselbalch Equation
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Consider a buffer solution that is 0.50 M in NH3 and 0.20 M in NH4Cl. For ammonia, pKb = 4.75. Calculate the pH of 1.0 L of the solution upon the addition of 30.0 mL of 1.0 M HCl to the original buffer solution.
A
pH = 8.25
B
pH = 7.95
C
pH = 8.05
D
pH = 8.75
Verified step by step guidance1
Identify the components of the buffer solution: NH3 (ammonia) is the weak base, and NH4Cl provides the conjugate acid NH4+.
Use the Henderson-Hasselbalch equation for a buffer solution: \( \text{pH} = \text{pK}_\text{a} + \log \left( \frac{[\text{Base}]}{[\text{Acid}]} \right) \). First, convert \( \text{pK}_\text{b} \) to \( \text{pK}_\text{a} \) using \( \text{pK}_\text{a} = 14 - \text{pK}_\text{b} \).
Calculate the moles of NH3 and NH4+ initially present: \( \text{moles of NH3} = 0.50 \text{ M} \times 1.0 \text{ L} \) and \( \text{moles of NH4+} = 0.20 \text{ M} \times 1.0 \text{ L} \).
Determine the moles of HCl added: \( \text{moles of HCl} = 1.0 \text{ M} \times 0.030 \text{ L} \). HCl will react with NH3 to form NH4+, changing the concentrations of NH3 and NH4+.
Recalculate the concentrations of NH3 and NH4+ after the reaction with HCl, then substitute these values into the Henderson-Hasselbalch equation to find the new pH of the solution.
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