Student performance calculating the pH of a strong acid or strong base solution before and after instruction in general and analytical chemistry courses was investigated using open-response questions in which a pH equation was not provided. Prior to instruction, students in both classes scored very low. General chemistry students lacking prior knowledge to convert concentration to a pH value manipulated the provided numbers in different calculations and analytical chemistry students not activating their prior knowledge used inappropriate mathematical functions. After instruction gains were high in both classes, with the most prevalent error involving the stoichiometric dissociation of a strong base. These results indicate a relatively simple algorithm such as calculating the pH of a strong acid or base may be readily acquired during instruction but may not be strongly retained after the course finishes.