Indeed, Kw is the ionization expression of per number of moles, contrary to Ka that is the ionization expression per molarity (even better, normality) of water and therefore, pkw = 14 and pKa = 15.7.
In other words, Kw measures the water ionization degree, contrary to pKa that measures the acidity of water and helps the comparison with other compounds. But when being in mixtures and solutions, the ratio of ionized/noninized molecules of water, remains 10exp(-14) and therefore, Kw is the predominant and valuable constant, for further mathematical applications.
So, let’s do it again:
ROH + OH(-) → RO(-) + H2O
Kreaction = [RO(-)][H2O]/ [HO(-)][ROH]
If you multiply and divide that function by [H+], you finally get:
K = Ka(ROH diss.)/Kw
ATTENTION: The ratio of concentrations [RO(-)]/[HO(-)] per volume unit does not change, regardless the changes of concentrations of ROH and water that occured during the alkali exchange reaction.