This paper describes a first-year undergraduate exercise involving the synthesis of an "unknown" polyiodide salt, (NMe[subscript 4])[I[subscript 5]], and the determination of its formula using the classic iodine-thiosulfate (iodometric) titration. The exercise introduces students to simple synthetic and titrimetric techniques, assists the development of their chemical and glassware handling and titrimetric skills, and acquaints them with unfamiliar polyiodide compounds. It also introduces students to key chemical concepts such as The Law of Definite Proportions, percentage elemental composition, empirical chemical formulas, quantitative stoichiometric reactions, and halogen catenation.