# I stumped the AP Chemistry and AP Calculus teacher: Where are the units in this calculation?

Lots of reactions occur in dilute solutions, where molarities work just fine. Not just as silly oversimplifications for students. Us actual chemists are happy to use them, depending on the reaction in question.

Certainly, moles (or raw numbers of particles, if you like scientific notation) are more convenient than grams for gases, because of the ideal gas law. But suppose that you have a sample of some unknown white powder, and you want to identify it, and the first step of your test is to dissolve some of it in water. In that case, you’d have to measure grams (or kilograms or slugs or carats or whatever) of your powder, not moles, because you don’t yet know what its molar mass is.

Agreed Ruken. Molarities work just fine once you understand the concept of molarities and normalities.

The point I am trying to make is that students get unnecessarily confused when you try to teach reaction rates (and thermodynamics) lumped with the concept of molarities and molalities etc. Instead, I am saying that students should first understand reaction kinetics (first order, second order, etc) based on mole fraction and then extend the learnings to molarities / normalities etc.

Add to the above mix, the Arrhenius Equation k=A exp(E/RT) , the units get more and more complicated when using molarities.

Agreed Chronos. In today’s time, you will send the sample for Mass Spectroscopy or Chromatography depending on the properties but you are right the first step will be to measure mass in a solution. You may measure the pH of the solution which will also give you an indication (not confirmation)of the molarity.

(Just so we are on the same page, the first test probably will be a flame test in olden days to see the color of the flame.)

If ‘Amazon Box’ is incorporated into the next SI update, I know who to blame.