Here's how to calculate the hydroxide ion concentration (OH-) if the pH is 8:

1. Relationship between pH and pOH

* pH + pOH = 14

2. Calculate pOH

* pOH = 14 - pH

* pOH = 14 - 8

* pOH = 6

3. Calculate [OH-]

* [OH-] = 10^-pOH

* [OH-] = 10^-6

* [OH-] = 1 x 10^-6 M

Therefore, the hydroxide ion concentration ([OH-]) is 1 x 10^-6 M when the pH is 8.