The measured pOH of a solution is 5.5. What is the hydrogen ion, [H+ ],
concentration?

First, calculate pH using the pOH:

14 - pOH = pH --> pH = 14 - 5.5 = 9.5

[H+] = 10^-pH = 10^-9.5 = 3.2 x 10^-10 M ≈ 3 x 10^-10 M (The answer should technically only be one sig fig, since the pOH is given to only one decimal place).

oxygen, selenium, tellurium, and polonium.

they're all from the same group as sulfur (group 16).