Calculate the pH of the solution made by adding 0.50 mol of HOBr and 0.30 mol of KOBr to 1.00 L of water. The value of Ka for HOBr is 2.0 × 10⁻⁹. What is the pH of the solution?

• A. 9.26
• B. 4.74
• C. 3.00
• D. 7.00

Answer: (B)

Verified step by step guidance

1. Identify the components of the solution: HOBr is a weak acid and KOBr is its conjugate base. This is a buffer solution.
2. Use the Henderson-Hasselbalch equation to find the pH of the buffer solution: \( \text{pH} = \text{pKa} + \log \left( \frac{[A^-]}{[HA]} \right) \).
3. Calculate \( \text{pKa} \) using the given \( \text{Ka} \) value: \( \text{pKa} = -\log(\text{Ka}) \). Substitute \( \text{Ka} = 2.0 \times 10^{-9} \) into the equation.
4. Determine the concentrations of the acid \([HA]\) and the conjugate base \([A^-]\): Since the solution is 1.00 L, \([HA] = 0.50 \text{ M}\) and \([A^-] = 0.30 \text{ M}\).
5. Substitute the values of \( \text{pKa} \), \([A^-]\), and \([HA]\) into the Henderson-Hasselbalch equation to calculate the pH.