Hydronium ion is H3O(+)
pH is log 1 / [H3O(+)] = - log [H3O(+)]
Rainfall: pH = 5.0 = - log [H3O(+)] => [H3O(+)] = -antilog (- 5.0) = 10^-5
Water; pH = 7.0 = - log [H3O(+)] => [H3O(+)] = - antilog (-7.0) = 10^ -7
Divide hidronium conentration of rainfall by hidrodium concentration of water:
10^-5 / 10^-7 = 10^2 = 100.
Then, hidronium concentration in rainfall is 100 times hidronium concentration in water.
Answer: option a. 100 times.
