Question

What is the [h3o+] in a solution with a poh = 5.20?

Answer 1

POH scale is used to measure how alkaline a solution is
pOH can be calculated as follows
pOH = - log [OH⁻]
pOH is 5.20 and using this the pH can be calculated
pOH + pH = 14
pH = 14 - pOH 
pH = 14 - 5.20 = 8.80
pH scale is used to measure how acidic a solution is
pH = -log [H₃O⁺]
[H₃O⁺] = antilog (-8.80)
[H₃O⁺] = 1.6 x 10⁻⁹ M
answer is [H₃O⁺] = 1.6 x 10⁻⁹ M