We get the molar mass of a compound by summing up the masses of the individual atoms making the compound. As for ethane, (C2H6), the atomic mass of carbon atom is 12 while that of hydrogen atom is 1. The summation is as follows: (12*2+6*1)
Therefore one molecules of ethane has a mass of 30.
Three moles have a mass of 30*3=90a.m.u
