A buffer is prepared by dissolving 0.50 g of benzoic acid (C6H5COOH, molar mass = 122 g·mol−1) and 0.75 g of sodium benzoate (C6H5COONa, molar mass = 144 g·mol−1) in water to make 250 mL of solution. Given that the pKa of benzoic acid at 25 °C is 4.20, what is the pH of the buffer?
a) 4.20
b) 4.30
c) 4.10
d) 4.35