#2H_2OrightleftharpoonsH_3O^+ + HO^-#
And #K_w=[H_3O^+][HO^-]=10^-14# at #298*K#. This equilibrium has been extensively measured.
#"6. and 7"# We can take #log_10# of both sides...........
#log_10K_w=log_10[H_3O^+]+log_10[HO^-]#
#underbrace(log_10(10^-14))_(-14)=log_10[H_3O^+]+log_10[HO^-]#
And so on rearrangement.........
#14=-log_10[H_3O^+]-log_10[HO^-]#
But by definition........
#14=underbrace(-log_10[H_3O^+])_(pH)underbrace(-log_10[HO^-])_(pOH)#
And so #14=pH+pOH#, under standard conditions of #298*K# and near #1*atm#.
If #pH=7#, then, clearly, #[H_3O^+]=[HO^-]#, and the solution is NEUTRAL.
If #pH<7#, then, clearly, #[H_3O^+]>[HO^-]#, and the solution is ACIDIC.
If #pH>7#, then, clearly, #[H_3O^+]<[HO^-]#, and the solution is ALKALINE.
#8.# We gots #[H_3O^+]=3.5xx10^-8*mol*L^-1#.
And given the former....#pH=-log_10[H_3O^+]=-log_10(3.5xx10^-8)#
#=-(-7.46)=7.46#, i.e. a basic solution where #[H_3O^+]#.
#pOH=14-7.46=6.54#, and #[HO^-]=10^(-6.54)*mol*L^-1=2.85xx10^-7*mol*L^-1#.
And if we gots #[H_3O^+]=0.0065*mol*L^-1#, #pH=-log_10(0.0065)=-(-2.19)=2.19#.
I leave it to you to calculate #pH# and #pOH# given the defining relationship.....#14=pH+pOH#