We introduce a computer exercise that bridges spectroscopy and thermodynamics using statistical mechanics and the experimental data taken from the commonly used laboratory exercise involving the rotational-vibrational spectrum of HCl. Based on the results from the analysis of their HCl spectrum, students calculate bulk thermodynamic properties such as internal energy, absolute entropy, molar enthalpy of formation, and Gibbs energy of formation at 25 [degree]C. This exercise serves as an opportunity for students to review the vibrational-rotational spectrum of HCl and to put in to practice their knowledge of statistical mechanics. The results show excellent agreement with values found in most introductory physical chemistry texts. This is a nice teaching tool because it uses experimental data to demonstrate the inter-relationship between quantum mechanical states and the bulk thermodynamic properties of gases. (Contains 1 table.)