Answer: The way to get the answer is to know and understand certain relationships between [H3O+] and [OH-]. One such relationship which you must know (memorize) is...
[H3O+] [OH-] = 1x10-14 = Kw (the rest is algebra)
[H3O+] [1x10-12] = 1x10-14
[H3O+] = 1x10-14 / 1x10-12
[H3O+] = 1x10-2 = 0.01 M
Explanation: HOPE IT HELPS!
