Final answer:
To calculate [OH-] given [H3O+], use the water dissociation constant, Kw = [H3O+][OH-]. A solution is acidic if [H3O+] > [OH-], basic if [H3O+] < [OH-], and neutral if both ions are at 1 x 10^-7 M.
Step-by-step explanation:
Calculating Hydroxide and Hydronium Ion Concentrations
To calculate the hydroxide ion concentration, given the hydronium ion concentration, you can use the water dissociation constant (Kw = 1.0 × 10-14 at 25°C). The relationship between [H3O+] and [OH-] is given by the equation Kw = [H3O+][OH-]. Therefore, if you have a [H3O+] of 2.0 × 10-3 M, the [OH-] can be calculated as follows:
[OH-] = Kw / [H3O+] = 1.0 × 10-14 / 2.0 × 10-3 = 5.0 × 10-12 M
A solution is acidic if the [H3O+] > [OH-], basic if [H3O+] < [OH-], and neutral if [H3O+] = [OH-] = 1 × 10-7 M. Therefore, given that [OH-] is smaller than [H3O+], the solution is acidic.
As an example, if you're provided with an [OH-] of 4.0 × 10-4 M, the corresponding [H3O+] would be calculated:
[H3O+] = Kw / [OH-] = 1.0 × 10-14 / 4.0 × 10-4 = 2.5 × 10-11 M, indicating that the solution is basic, since [H3O+] is less than [OH-].