Final answer:
The concentration of hydroxide ion in the solution is 2.38 x 10^-7 M.
Step-by-step explanation:
The concentration of hydroxide ion in a solution can be determined using the equation [H3O+] x [OH-] = Kw, where Kw is the ion product constant for water at 25 °C, equal to 1.0 x 10^-14 M^2. In this case, the concentration of hydroxide ion ([OH-]) can be calculated as:
[OH-] = Kw / [H3O+]
Substituting the given [OH-] value of 4.2 x 10^-8 M:
[OH-] = (1.0 x 10^-14 M^2) / (4.2 x 10^-8 M)
= 2.38 x 10^-7 M