For strong acids and bases after dilution, you do: volume before dilution x concentration before dilution = moles, then moles / volume after dilution to get concentration of either H
+ or OH
- after dilution. Then -log [H
+] to get pH. If it's a base, then do [H
+] = Kw/[OH
-] first to get [H
+]. If it's a diprotic acid or base, such as H
2SO
4 or Ba(OH)
2, remember to multiply moles of acid or base by 2 to get the number of moles of H
+ or OH
-.
When mixing acids and bases, first do concentration x volume to calculate the number of moles of acid and base to find which one is in excess (remember to check if either the acid or base is diprotic), then find the difference between the number of moles of H
+ and OH
- and use -log [H
+] to find the pH. If the excess is a weak acid HX then, you need to use K
a = [H
+]
2/[HX], and if it's a buffer solution, then you need to use K
a = [H
+][X
-]/[HX], where [X
-] is the concentration of the salt, in order to find [H
+].
I hope this makes sense.