To find the pH of Buffer A after 5.0 mL 6.0 M NaOH has been added, first calculate how many moles HONH₃⁺ are present in the buffer and how many moles NaOH are added. Then calculate how many moles NaOH will be left after the reaction. Convert this to the concentration OH^- present by dividing by the final volume of the solution. Calculate [H⁺] using K_w and then calculate the pH of the solution: