You spill 1500 mg of rubbing alcohol in a box that measures 4 m x 2m x 3m. If the entire sample evaporates into a gas, what is the concentration of the alcohol in milligrams per cubic meter?

You must be over thinking this problem.

What's the volume of the box? That's
4m x 2m x 3m = 24 m^3

So the concn in the box must be 1500 mg/24 m^3 = ?