Chemistry - acid/base

Question

Assume there are aqueous solutions of the same concentrations. Please place the following compounds in order of increasing pH. K₂HPO₄, H₃AsO₄, LiHSO₃, NaHC₂O₄, C₆H₅N, C₂H₅NH₃NO₃, NH₄ClO₄

I think I already understand that:

K₂HPO₄ is a salt made from a strong base + weak acid --> basic solution

LiHSO₃ is a salt made from a strong base + weak acid -- > basic solution

NaHC₂O₄ is a salt made from a strong base + weak acid --> basic solution

C₆H₅N is a weak base --> basic solution

C₂H₅NH₃NO₃ is a salt made from a weak base + strong acid --> acidic solution

NH₄ClO₄ is a salt made from a weak base + strong acid --> acidic solution

H₃AsO₄ is a weak acid --> acidic solution

Please help me order them according to pH. I see I have 3 acidic solutions and 4 basic solutions, but do not know how to put them in order. From what I understand, the weaker the original acid, the stronger the conjugate base in the salt, and the weaker the original base, the stronger the conjugate acid in the salt.

Answer 1

Yep, you are correct in all that you stated, assuming all are present at the same concentration. In order to arrange them successfully, you'll need to know the Ka/Kb or pKa/pKb for the different acids/bases.

For example, to arrange the acidic solutions, you have C₂H₅NH₃NO₃, NH₄ClO₄, and H₃AsO₄. The Ka for H₃AsO₄ is somewhere around 10^-3, which is considerably stronger than the Ka for either C₂H₅NH₃⁺ or NH₄⁺.

So H₃AsO₄ would have the lowest pH. Next, compare Ka for the other 2. I believe that NH₄⁺ is stronger than C₂H₅NH₃⁺. If you can't find Ka for C₂H₅NH₃⁺, find Kb for C₂H₅NH₂ and calculate Ka (KaKb = Kw). So, the order for increasing pH among the acids would be H₃AsO₄ < NH₄⁺ < C₂H₅NH₃⁺.

You'd have to do the same for the basic salts.

Hope this helps.