assuming equal concentrations rank these solutions by pH

Question

HBrO(aq)

HCIO4(aq)

Sr(OH)2(aq)

NH3(aq)

RbOH(aq)

Answer 1

Hi Makenzie,

 

To determine pH from acids, you simply need to note which is the stronger acid. Inmost introductory chemistry classes, we use a list of strong acids (those which completely dissociate into H⁺ and the anion). Typically it is HCl, HBr, HI, HNO₃, H₂SO₄ (1st H⁺ only), and HClO₄. Every other acid is considered a weak acid, which means it only slightly dissociates into H⁺ and the anion. For equal concentrations, the strong acid will have a lower pH than the weak acid, so that takes care of two entries on your list - but what about the bases?

 

Metal hydroxides completely dissociate into the cation and OH^- when they dissolve, and the more OH^- in the original salt the more OH^- in the solution. [H⁺] and [OH^-] vary inversely, so the more hydroxide in the solution the less H⁺, which means more OH^- is a higher pH.

 

The only compound left is the ammonia, which does not have any hydroxides of its own, but is still a base. Because of the lack of hydroxide, it must produce them from water making ammonia a weak base, so the pH of its solution will be lower than the metal hydroxides, but higher than the acids.

 

Jim S.