What is the pH of a solution formed by mixing 250.0 mL of 0.15 M NH4Cl with 200.0 mL of 0.12 M NH3, given that the Kb for NH3 is 1.8 × 10^-5?

• A. 7.00
• B. 8.95
• C. 5.05
• D. 9.25

Answer: (B)

Verified step by step guidance

1. Calculate the moles of NH4Cl and NH3 in their respective solutions. Use the formula: \( \text{moles} = \text{concentration} \times \text{volume} \).
2. Determine the total volume of the mixed solution by adding the volumes of the NH4Cl and NH3 solutions.
3. Calculate the concentrations of NH4Cl and NH3 in the mixed solution using the formula: \( \text{concentration} = \frac{\text{moles}}{\text{total volume}} \).
4. Use the Henderson-Hasselbalch equation for a buffer solution: \( \text{pH} = \text{pK}_a + \log \left( \frac{[\text{base}]}{[\text{acid}]} \right) \). First, find \( \text{pK}_a \) using \( \text{pK}_a = 14 - \text{pK}_b \), where \( \text{pK}_b = -\log(\text{K}_b) \).
5. Substitute the concentrations of NH3 (base) and NH4Cl (acid) into the Henderson-Hasselbalch equation to find the pH of the solution.