Question

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.

SPECIES	RELATIVE pH OF 0.1M AQUEOUS SOLUTION
H2C2O4	3
HNO2	4
HC2O4-	CHOOSE ONE
IO3-	CHOOSE ONE
NO2-	CHOOSE ONE
H3O+	CHOOSE ONE (should be lowest)
HIO3	2
H2O	5

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

Answer 1

The stronger the acid, the weaker its conjugate base (and vice-versa). Considering this and the ones for which the ranking is given, it's just a matter of mirroring the position, considering the conjugate acid or base. For example: H C204 is given as the most acidic compound, which means that its base (HC204) will be the weakest of the bases, making it the one that will result in the lowest pH, among the bases. SPECIES RELATIVE PH OF 0.1M AQUEOUS SOLUTION H2C204 HNO2 HC204 103 NO2 H3O+ HIO3 H2O