Stoichiometry has always been a puzzling subject. This may be partially due to the way it is introduced to students, with stoichiometric coefficients usually provided in the reaction. If the stoichiometric coefficients are not given, students find it very difficult to solve problems. This article describes a simple 4-h laboratory experiment for the determination of aluminum content in commercial samples through the stoichometric relationship between aluminum and released hydrogen gas. The experiment was performed by high-school students at an advanced preparation course for the Chemistry Olympiad. The aluminum content in foil samples was determined by measuring the amount of formed hydrogen gas through mass balance of the reaction system. The experiment is appropriate for high-school chemistry or a first-semester general chemistry course.