Question - A biochemist has 100 mL of a 0.10 M solution of a weak acid with a pKa of 6.3. She adds 6.0 mL of 1.0 M HCL, which changes the pH to 5.7. What was the pH of the original solution? Solutions manual in Lehninger Biochem textbook says that the answer is pH=6.9. I do not know how you arrive at this answer!