A solution is found to have a pH of 8.3. What is the hydroxide ion, [OH-]
concentration?

[OH-] = 2 x 10^-6 M

Explanation:

Calculate pOH first: 14 - pH = pOH --> pH = 14 - 8.3 = 5.7

[OH-] = 10^-pOH = 10^-5.7 = 2 x 10^-6 M

Only one sig fig in the concentration, because the pH only had one decimal place.