An equal amount of each of the following salts is added separately to the same volume of water.

Which salt will have the greatest effect on the pH of water?

A.     \({\text{Al(N}}{{\text{O}}_{\text{3}}}{{\text{)}}_{\text{3}}}\)

B.     \({\text{N}}{{\text{a}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}\)

C.     RbCl

D.     KBr

A

Students found this question to be somewhat difficult with 41.96% correct answers. The salt in each of the other three choices is neutral (made up of strong acid and a strong base) whereas the effect of the high charge density of the aluminium ion in the complex ion formed, \({{\text{[Al(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{6}}}{\text{]}}^ + }\), produces an acidic solution.