estimate.
1. pH = -log [H3O+] ---> [H3O+] = 10^-pH (I assume, you have a good grasp on logarithm)
say, [H30+] = 10^-6 and the question is asking you to find pH
compare the second equation
therefore, pH = 6
2. say [H+] = 2.1 x 10^-9, what's the pOH?
2.1 is ~2 x 10^-9, which is b/w 10^-8 (if the concentration is 10x10^9)and 10^-9 (if the concentration is 1x10^-9)
now based on the above equation, we can say that the pH must be b/w 8 and 9
but our concentration is near to 1x10^-9, therefore, our pH must be near 9. Say, 8.8?
We're still not done yet. the question is asking for a pOH.
pH + pOH = 14
8.8 + pOH = 14
pOH = 5.2
hope this makes sense
