This study investigated the strategies students in general chemistry and analytical chemistry courses used before and after instruction to solve a volumetric analysis problem in which the molar ratio was not 1:1. Questions like this have historically been very challenging for high school and undergraduate students. Student writing on open-response problems was analyzed to determine the problem-solving approach and identify errors made during the calculation. The two most prevalent strategies on pretests and post-tests were calculating mole amounts and using the equation M[subscript 1]V[subscript 1] = M[subscript 2]V[subscript 2]. Performance was significantly lower for students using the M[subscript 1]V[subscript 1] = M[subscript 2]V[subscript 2] as they often assumed a 1:1 molar ratio. The M[subscript 1]V[subscript 1] = M[subscript 2]V[subscript 2] heuristic was very popular despite its exclusion from formal instructional resources such as the textbook or lecture, and teachers seeking to improve instruction in this area must account for students using an approach not introduced in the classroom.