"This solution contains 71 mole percent of H2O"
I was recently at a Shell gas station pumping gas when I noticed the inscription "this gasoline may contain up to 10% by weight ethanol" on a label next to the digital display. Does this statement provide information that is directly useful to anyone, in particular, to consumers who have no conceptual basis in Chemistry whatsoever? What is the real utility of describing something by weight percent, as opposed to volume percent. As far as Chemistry is concerned, it seems that a "mole percent" basis would be more appropriate as it would entail the more important description of just how much ethanol there are in terms of relative numbers of molecules. This would at least elucidate the notion of whether the solution contains more of one molecule compared to the rest of the constituents.
Volume percent is actually a tricky calculation, since volumes are not always additive. For instance, if you mix 10 ml of methanol with 90 ml of water, you won't get 100 ml of solution.
Dick (Comment this)