While this is obviously not true, the amount of error that it introduces into the description of gases is slight and will generally be of little importance. The masses of the real gases are also generally quite small. It seems that it would be appropriate for them to also have mass values assigned at zero in order to simplify the descriptions of gases. This assumption would create a significant problem for chemists. One of the aspects of gas behavior that must be considered is temperature.
Temperature is a very useful variable when describing gases.
Temperature is related to Kinetic Energy. If the mass of gas particles was assumed to be zero, then the kinetic energy would calculate out as zero. Since gas phase systems can exist over a very wide range of temperatures, it would be a major error to treat all real gases as if they had zero temperature. The amount of error this would introduce into descriptions of gases with significant temperature values would be to great to be ignored. So, the Ideal Gas approximates all real gases by having zero volume and the mass of the gas particles is represented by the atomic mass or molecular mass of the substance.