Question

how many a) H^+ and OH^- ions are present in 250 ml of a solution of pH 3?

Answer 1

pH = -log (H^+)

(H+) = 1E-3 mols/L so in 250 mL is 1/4 that.
(H^+)(OH^-) = 1E-14
Solve for (OH^-) = ? in mols/L so in 250 mL is 1/4 that.
Then for each remember that 1 mol of anything has 6.02E23.
Post your work if you get stuck.