Final answer:
To find the [OH-] in a solution with [H+] = 4.2 × 10^-3 M, divide the ion-product constant of water (Kw = 1.0 × 10^-14) by [H+], which gives [OH-] = 2.38 × 10^-12 M, indicating an acidic solution.
Step-by-step explanation:
To determine the hydroxide ion concentration ([OH-]) in a solution given the hydrogen ion concentration ([H+]),
we use the ion-product constant for water (Kw), which is 1.0 × 10-14 at room temperature.
For a solution with [H+] = 4.2 × 10-3 M,
we can find [OH-] by rearranging the formula Kw = [H+]×[OH-] to [OH-] = Kw / [H+].
Substituting the known values, we get [OH-] = 1.0 × 10-14 / 4.2 × 10-3 M,
which calculates to approximately 2.38 × 10-12 M.
Since the [OH-] is less than the [H+], the solution is acidic.