A biochemist prepares two stocks of sodium phosphate buffer by titrating phosphoric acid with sodium hydroxide. She prepares a 0.10 M solution of sodium phosphate buffer at pH 2.15, and she prepares a 0.10 M solution of sodium phosphate buffer at pH 7.20. If she mixes 48 mL of the pH 2.15 buffer with 31 mL of the pH 7.20 buffer, what will be the pH of the resulting solution? Recall that the pKas of phosphoric acid are 2.15, 7.20, and 12.35.