Counterfeiting and adulteration of prescription drugs, herbal products, and food supplements are a global challenge, causing serious economic loss to drug marketers and health implications for humans. Accordingly, accurate determination of the purity of pharmaceuticals is critical for the quality assurance of prescription drugs. Herein, the first purity analysis of pharmaceuticals naproxen and propranolol in a guided-inquiry laboratory experiment (GILE) setting as part of an undergraduate instrumental analysis laboratory course is reported. The students were able to independently design analytical procedures to determine the purity and percentage composition of suspected adulterated naproxen samples with minimal supervision of the instructor. The results of the determined percentage compositions of naproxen by the students favorably compared with the known compositions of naproxen in their GILE samples provided by the instructor, with a root-mean-square percent relative error of 3.24%. The majority of the students were excited, motivated, and preferred the GILE to traditional laboratory experiments, in which students simply follow the lab manual. In addition, this GILE promoted the spirit of teamwork and challenged the students' critical thinking and problem-solving skills ability. The GILE also provided an opportunity for students to better understand the concepts and the practical utility of multiple analytical techniques for solving real-world problems and to experience typical challenges often encountered in the laboratory during chemical analysis and to determine strategies for resolving those challenges.