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 addition of 30.0 mL of 1.0 M HCl to the original buffer solution.

• A. pH = 9.25
• B. pH = 8.75
• C. pH = 8.95
• D. pH = 9.05

Answer: (B)

Verified step by step guidance

1. Identify the components of the buffer solution: NH3 (ammonia) is the weak base, and NH4Cl provides the conjugate acid NH4+. The initial concentrations are 0.50 M for NH3 and 0.20 M for NH4+.
2. Calculate the moles of NH3 and NH4+ in the buffer solution. Since the volume of the solution is 1.0 L, the moles of NH3 are 0.50 mol and the moles of NH4+ are 0.20 mol.
3. Determine the moles of HCl added to the solution. With a concentration of 1.0 M and a volume of 30.0 mL (0.030 L), the moles of HCl are 0.030 mol.
4. When HCl is added to the buffer, it reacts with NH3 to form NH4+. The reaction is: NH3 + HCl → NH4+ + Cl-. Calculate the new moles of NH3 and NH4+ after the reaction. Subtract 0.030 mol from the initial moles of NH3 and add 0.030 mol to the initial moles of NH4+.
5. Use the Henderson-Hasselbalch equation to calculate the pH of the buffer solution: \( \text{pH} = \text{pK}_\text{a} + \log \left( \frac{[\text{Base}]}{[\text{Acid}]} \right) \). First, find \( \text{pK}_\text{a} \) using \( \text{pK}_\text{a} = 14 - \text{pK}_\text{b} \). Then, substitute the concentrations of NH3 and NH4+ into the equation to find the pH.