In water, autoprotolysis occurs, and #[H_3O^+][HO^-]=10^-14# under standard conditions.
If we take #-log_10# of both sides of the expression:
#-log_10[H_3O^+]-log_10[HO^-]=-log_(10)10^-14#
But #-log_(10)10^-14=14# by definition, because #log_aB# is the power to which we raise the base #a# to get #B#.
Also by definition, #pH=-log_10[H_3O^+]# and #pOH=-log_10[HO^-]#.
Thus #pH+pOH=14#
At neutrality, #[HO^-]=[H_3O^+]#, and thus #pH=pOH=7#, that is the concentrations of hydroxide ion and hydronium ions are EQUAL. If #pH<7#, the solution is acidic, and #pH>7#, the solution is basic. Your #pH=9#, which suggest what with respect to #[HO^-]#?
It is worthwhile getting these definitions right. They are fairly simple and logical, and problems based on #pH# can become routine. If you have a problem, or if I have glossed over a step you do not understand, ask for clarification and someone here will help you.
