How do you find pH and OH of a solution?

Question

You have an aqueous solution with [H3O+] = (9.228x10^-5) M at 25°C. What is [OH-] and pH of this solution.

Answer 1

There are several possible ways to go about answering this question.

You should be familiar with the following relationships:

pH + pOH = 14

[H3O⁺][OH-] = 1x10^-14

pH = -log [H3O⁺]

pOH = -log [OH-]

From the given information that [H3O⁺] = 9.228x10^-5 M, we can find the pH directly:

pH = -log [H3O⁺]

pH = -log 9.228x10^-5

pH = 4.03

We can find the [OH-] in two different ways

i). [H3O⁺][OH-] = 1x10^-14

(9.228x10^-5)(OH-) = 1x10^-14

[OH-] = 1.08x10^-10 M

ii). pH + pOH = 14

pOH = 14 - pH

pOH = 14 - 4.03

pOH = 9.97

[OH-] = 1x10^-9.97

[OH-] = 1.07x10^-10