Henry's law Numerical
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. ;)