To solve this question, we need to use the following formula:
pH = - log[H3O+] or pOH = -log [OH-]
and pH = 10 - pOH or pOH = 10 - pH
Step 1: Let's use first
pOH = 10 - pH
pOH = 10 - 6.67
pOH = 3.33
Step 2: Let's use pOH = -log [OH-]
pOH = -log [OH-] is equal to 10^-pOH = [OH-]
so we have pOH = 3.33
10^-3.33 = [OH-]
[OH-] = 4.68 x 10^-4 mol/L
Answer: [OH-] = 4.68 x 10^-4 mol/L