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 0.010 mol of solid NaOH to the original buffer solution.

• A. 8.75
• B. 9.00
• C. 9.26
• D. 8.50

Answer: (C)

Verified step by step guidance

1. Identify the components of the buffer solution: NH3 (ammonia) and NH4Cl (ammonium chloride). NH3 is a weak base, and NH4+ is its conjugate acid.
2. Use the Henderson-Hasselbalch equation for a buffer solution: \( \text{pH} = \text{pKa} + \log \left( \frac{[\text{base}]}{[\text{acid}]} \right) \). First, calculate \( \text{pKa} \) from \( \text{pKb} \) using the relation \( \text{pKa} + \text{pKb} = 14 \). Therefore, \( \text{pKa} = 14 - 4.75 = 9.25 \).
3. Determine the initial concentrations of NH3 and NH4+ in the buffer solution. NH3 is 0.50 M and NH4+ is 0.20 M.
4. Calculate the change in concentration of NH3 and NH4+ upon addition of NaOH. NaOH will react with NH4+ to form NH3 and water: \( \text{NH4}^+ + \text{OH}^- \rightarrow \text{NH3} + \text{H2O} \). The amount of NaOH added is 0.010 mol, which will decrease the concentration of NH4+ by 0.010 M and increase the concentration of NH3 by 0.010 M.
5. Substitute the new concentrations into the Henderson-Hasselbalch equation: \( \text{pH} = 9.25 + \log \left( \frac{0.50 + 0.010}{0.20 - 0.010} \right) \). Calculate the logarithmic term to find the pH of the solution after the addition of NaOH.