In water the following equilibrium operates:
#2H_2O(l) rightleftharpoonsH_3O^+ + HO^-#
As for any equilibrium, we can write the equilibrium expression:
#([H_3O^+][HO^-])/[[H_2O]]# #=# #K'_w#
Since #[H_2O]# is huge, we can simplify this expression:
#[H_3O^+][HO^-]# #=# #K_w#
At #298K#, #[H_3O^+][HO^-]# #=# #K_w=10^(-14)#
Taking #log_10# of both sides:
#log_10[H_3O^+] + log_10[HO^-]# #=# #log_10(10^-14)# #=# #-14#
Now we can define #pH# #=# #-log_10[H_3O^+]#, and
#pOH# #=# #-log_10[HO^-]#.
And thus #pOH +pH=14# #"(finally!)"#
Thus #pOH# of #1xx10^-4mol*L^-1" NaOH"# #=# #-log_10(1xx10^-4)=4#
Since #pOH +pH=14#, #pH=10#, as required.