Here's how to calculate the hydroxide ion concentration ([OH-]) of a solution with a pH of 8.57:

1. Understand the Relationship between pH and pOH

* pH + pOH = 14

2. Calculate the pOH

* pOH = 14 - pH

* pOH = 14 - 8.57 = 5.43

3. Calculate the Hydroxide Ion Concentration ([OH-])

* [OH-] = 10^-pOH

* [OH-] = 10^-5.43

* [OH-] ≈ 3.72 x 10^-6 M

Therefore, the hydroxide ion concentration of a solution with a pH of 8.57 is approximately 3.72 x 10^-6 M.