because we are given the concentration of the hydroxide ion(OH⁻), we will first find the the concentration of [H⁺] ions and find the pH from there.
Finding the concentration of H⁺ ions:
we know the relation:
[OH⁻]*[H⁺] = 10⁻¹⁴ [square brackets denote the concentration]
(10⁻⁴)[H⁺] = 10⁻¹⁴
[H⁺] = 10⁻¹⁰ [multiplying both sides by 10⁴]
Finding the pH:
pH = -log[H⁺]
pH = -log(10⁻¹⁰)
pH = 10
