Question

Why are average kinetic energy of hydrogen, carbon dioxide and ethane same at same temperature?

Answer 1

The average kinetic energy of two different samples of gas will be the same if the two samples are at the same temperature. But the molecules in either sample will have a distribution of speeds and therefore a distribution of kinetic energies (some molecules will be moving faster and some slower). So the average kinetic energy between two different samples is the same but the individual molecules in the samples will not each have the same exact kinetic energy.