pH is used to measure the acidity of a substance and it can be used to calculate the amount of H+ ions in a solution. pH is expressed as:
pH = -log [H+]
For a normal rain:
5.5 = -log [H+]
[H+] = 1 x 10^-5 M
For an acid rain:
1.5 = -log [H+]
[H+] = 0.03 M