Order these chemical species by increasing pH of an 0.1M aqueous solution of each. That is, imagine making an 0.1M solution of each species. Select 1 next to the species that makes the solution with the lowest pH. Select 2 next to the species that makes the solution with the next higher pH, and so on. Notice that some of the rankings have been filled in for you already. Also notice that water is on the list. For that particular case, just compare the pH of pure water to the pH of the other solutions.

Note for advanced students: for all charged species, you may assume the necessary counterions act as neither acids nor bases.

In bold are preset correct answers. I believe that hydronium should always be 1 or “the lowest”.