1.     10^-pH = 10^-6 moles H⁺/L

2.     The very small amount comes from the water itself.

3.     10^-3/ 10^-7  = 10 000 times more H⁺ at pH=3 than at pH = 7(neutral)

4.     See chart on p83 where rain is normally at pH = 5.6.

So if it is 10 times more acidic we go down 1 pH unit. That gives a pH of 4.6.