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.