[user11]

If we have a weak base reaction such as

CN(-) + H2O = HCN + OH(-)

We know that CN(-) is the base which is competing with the other base OH(-) for H(+). But since OH(-) is a stronger base than CN(-) most of the OH(-) will transform the produced HCN back into CN(-) again.

Here is my question: Could CN(-) and OH(-) also use H(+) from  H3O(+) produced by waters  autoionization? We know that this concentration at pH=7 is 10^-7 which is very small and would contribute very little to the weak base +water reaction at high concentrations of base. But at very small concentrations of base (close to 10^-7) I could se how the concentration of OH – and OH(+) could influence the equilibrium of the water base reaction.  So is this the reason why we always include water autoionization wen calculating small concentrations of weak acids and weak bases in water solution?