**Now My question is, why concentration is Considered only in Grams? shouldn't it be in Molarity, Mole fraction or Strength(gm/Vol) etc? **
They are implicitly assuming that the number of moles of ethane in solution is very small (in both cases) compared to the number of moles of the solvent. So, in both cases, the concentration of ethane and its mole fraction are essentially proportional to the mass of ethane in the solution and the amount of solvent is the same.
For this example, the volume of the solution doesn't matter. It's a thought experiment, doesn't say what sovent is used, and you could replace ethane by helium or miraculane.
As long as you talk about the same compound, mass is proportional to molar quantity. Plus the units cancel out in the calculation anyway.
Think of the hypothetic experimentator as a guy who only has one glass beaker, without graduation. ;)