Hello!
A solution of a weak acid and its conjugate base is called a
buffer solution. It can resist the addition of bases and acids while maintaining the pH approximately constant.
To calculate the pH of a buffer solution, we'll use the famous Henderson-Hasselbach equation, applying the pKa value of the Benzoic Acid (4,20). In this equation, [A⁻] represents the concentration of the Conjugate Base and [HA] the concentration of the acid.
![pH=pKa + log( ( [A^(-)])/([HA]) ) \\ \\ pH=4,20 + log ( (0,170 M)/(0,245) ) \\ \\ pH=4,04](https://img.qammunity.org/2019/formulas/chemistry/college/yxs1nkn7qjcrp21c5uwf6cw1dk1cqfr1ia.png)
So, the pH of the solution would be
4,04.Have a nice day!