Answer:
4.0
Explanation:
we have a buffer system formed by a weak acid (benzoic acid C₆H₅O₂H) and its conjugate base (benzoate ion C₆H₅O₂⁻, coming from sodium benzoate C₆H₅O₂Na). Given the acid dissociation constant (Ka) for benzoic acid is 6.5 × 10⁻⁵, we can calculate the pH of the buffer solution using the Henderson-Hasselbach equation.
pH =pKa + log [base]/[acid]
pH =-log Ka + log [C₆H₅O₂⁻]/[C₆H₅O₂H]
pH =-log 6.5 × 10⁻⁵ + log (0.15 M/0.25 M) = 4.0
