A method of analyzing a gas specimen mixture includes measuring the concentrations of inert components in the gas specimen mixture and the pressure and temperature of the gas specimen mixture. A number of sample gas mixtures are generated with varying percentages of hydrocarbon gases, each including the measured inert component concentrations. For each generated sample gas mixture, the method includes calculating the speed of sound therein based on the measured pressure and temperature and the particular percentages of hydrocarbon gases therein, measuring the speed of sound in the gas specimen mixture, and iteratively comparing the measured speed of sound with the calculated speed of sound in different sample gas mixtures until convergence for a particular sample gas mixture. The molecular weight of the particular sample gas mixture is calculated, and set the molecular weight of the gas specimen mixture to the calculated molecular weight.