What is the pH of a solution made by mixing 0.30 mol NaOH, 0.25 mol Na2HPO4, and 0.20 mol H3PO4 with water and diluting to 1.00 L?

• A. pH = 5.0
• B. pH = 3.0
• C. pH = 13.0
• D. pH = 7.0

Answer: (C)

Verified step by step guidance

1. Identify the species in the solution: NaOH is a strong base, Na2HPO4 is a salt that can act as a buffer, and H3PO4 is a weak acid.
2. Calculate the concentration of each component in the solution by dividing the moles by the volume of the solution (1.00 L). For example, the concentration of NaOH is \( \frac{0.30 \text{ mol}}{1.00 \text{ L}} = 0.30 \text{ M} \).
3. Determine the dominant reaction in the solution. NaOH will react with H3PO4 to form water and Na2HPO4, neutralizing the acid. Write the balanced chemical equation: \( \text{NaOH} + \text{H}_3\text{PO}_4 \rightarrow \text{Na}_2\text{HPO}_4 + \text{H}_2\text{O} \).
4. Calculate the remaining concentration of NaOH after the reaction with H3PO4. Since NaOH is in excess, subtract the moles of H3PO4 from the moles of NaOH to find the concentration of unreacted NaOH.
5. Use the concentration of unreacted NaOH to calculate the pH of the solution. Since NaOH is a strong base, the pH can be found using the formula \( \text{pH} = 14 - \text{pOH} \), where \( \text{pOH} = -\log[\text{OH}^-] \).