The molecular weight of a mixed gas has been classified as one of the most important experimental subjects for the first grade students in the Kisarazu National College of Technology, because they can learn the property of gas as well as the meaning of mole. Recently, it has been found that the average molecular weight of the mixed gas obtained from a gas lighter bomb, which contains propane and butane, scatters in a considerably wide range. A study showed that the average molecular weight depends on the weight of the gas bomb, that is the weight of gas left. It is obvious that either one of two components is likely to evaporate due to the difference of vapor pressure. This paper shows how a new gas mixture such as air is adopted to eliminate intrinsic errors as in the case of the gas mixture of propane and butane, and also discuss on some subjects to be solved, comparing with the former method.