Determination of the Molar Mass of an Unknown Gas with a 2-L Soda Bottle

This laboratory experiment was developed to provide a safe, economical, and effective way to instruct high school or undergraduate general chemistry students in the use of the ideal gas equation to determine the molar mass of an unknown gas using a balance, a modified 2-L soda bottle, and a vacuum source. The experiment was performed by approximately 145 student participants. We assessed the effectiveness of the laboratory by three separate methods: a qualitative pre/post exercise student self-assessment, a quantitative pre/post exercise quiz, and the results of the laboratory experiment. For the qualitative survey, many of the critical concept questions showed a statistically significant (p-value < 0.05 in an unpaired homoscedastic t-test) change from pre- to post-survey, indicating an improved understanding. Similarly, the average on the quantitative pre/post 4-point quiz increased from 1.59 ± 1.21 (sdev) to 2.43 ± 1.21 (sdev), and demonstrated a statistically significant improvement (p-value < 0.001). Finally, 68.1% of the students correctly determined the molar mass of the unknown gas within ± 2 g/mol, and an additional 6.8% made the determination between 2 ± g/mol and ± 4 g/mol. Thus, by all assessment methods we undertook, students were able to perform this experiment and learn from it.