1L soulution has a pH of 6
how many OH- would it tak eto make it a pH7 solution?
pH = 6 = (H^+) = 1 x 10^-6M and OH^- = 1 x 10^-8
You want pH = 7 = (H^+) = 1 x 10^-7 and OH^- = 1 x 10^-7
In 1 L you have 1 x 10^-8 moles and you want 1 x 10^-7 moles so you must add
1 x 10^-7 - 1 x 10^-8 = 9 x 10^-8 mols and that times 6.022 x 10^23 OH^- ions/mole = number of OH ions to add. Check my thinking. Check my arithmetic.