What is the pH of a solution of 0.20 M HNO2 containing 0.10 M NaNO2 at 25°C, given the Ka of HNO2 is 4.5 × 10⁻⁴?

• A. 4.25
• B. 3.55
• C. 3.35
• D. 2.85

Answer: (B)

Verified step by step guidance

1. Identify that the problem involves a weak acid (HNO2) and its conjugate base (NO2⁻ from NaNO2), which suggests the use of the Henderson-Hasselbalch equation.
2. Write the Henderson-Hasselbalch equation: \( \text{pH} = \text{pKa} + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \), where \([\text{A}^-]\) is the concentration of the conjugate base and \([\text{HA}]\) is the concentration of the weak acid.
3. Calculate the pKa from the given Ka of HNO2: \( \text{pKa} = -\log(4.5 \times 10^{-4}) \).
4. Substitute the concentrations of the conjugate base \([\text{A}^-] = 0.10 \text{ M}\) and the weak acid \([\text{HA}] = 0.20 \text{ M}\) into the Henderson-Hasselbalch equation.
5. Solve the equation to find the pH of the solution.