K 1 -1 which is just what we had for the inverse reaction before. This looks worse than it is. All it takes is to remember the various conventions underlying the mass action law, something you will get used to very quickly in actual work. If you think about it, that happens whenever the number of mols on both sides of a reaction equation is not identical. This is a tricky point and it is useful to illustrate it.
Lets construct some examples.
We take one reaction where the mol count changes, and one example where it does not. Now let the reaction take place until equilibrium is reached. Now for the second example. O which formally describes the formation of ozone, albeit a chemist might shudder at that reaction. We had several ways of measuring concentrations, and it is quite illuminating to look closely at how they compare for our specific examples.