Using an easy-to-make cylindrical resonator, students can measure the speed of sound in a gas, u, with sufficiently high precision (by locating standing-wave Lissajous patterns on an oscilloscope) to observe real gas properties at one atmosphere and 300 K. For CO[subscript 2] and SF[subscript 6], u is found to be 268.83 and 135.25 m s[superscript -1], respectively, versus their ideal-gas (zero-pressure) values of 270.15 and 136.67 m s[superscript -1]. These differences arise from intermolecular interactions. It is shown that if the molar isobaric heat capacities, C[subscript p], m, are calculated from the one atmosphere u values assuming perfect gas behavior, significant errors are produced. For CO[subscript 2] and SF[subscript 6] these errors are 2.7% and 25%, respectively. Experimental C[subscript p], m values are compared with those obtained based on the equipartition theorem. (Contains 1 figure and 2 tables.)