What is the concentration of hydroxide ions in 1 liter of beer?

The concentration of Hidroxide ions in 12 liter of pure water is:

1 * 10^ (-7) moles.

And we know that for water: pH = 7.

Each pH unit has 10 times as many Hidroxide ions as unit above. For beer: pH = 4. Therefore pure water (pH = 7) has 1000 times more ions than beer (pH = 4).

1 * 10 ^ (-7) : 1000 = 1 * 10^ (-7) : 10^ (-3) = 1 * 10^ (-10)

Answer: 1 * 10^ (-10) moles of Hydroxide ions.