Computational chemistry undergraduate laboratory courses are now part of the chemistry curriculum at many universities. However, there remains a lack of computational chemistry exercises available to instructors. This exercise is presented for students to develop skills using computational chemistry software while supplementing their knowledge of physical organic chemistry, specifically Hammett plots. Students use quantum calculations to determine the gas-phase Gibbs energy of ionization of substituted benzoic acids and phenols to generate a theoretical Hammett plot, thereby familiarizing themselves with the application of computational chemistry to physical organic chemistry. (Contains 1 scheme, 1 table, and 2 figures.)