Match the acid/base pair that is suitable for preparing a buffer of each given approximate concentration.?

Question

C6H5NH2/ C6H5NH3Cl

NaH2AsO3/Na2HAsO3

NH3/NH4Cl

[H3O+] = 10-4

[H3O+] = 10-9

[H3O+] = 10-7

no idea where to start! help!

Answer 1

the ideal buffer made with a weak acid has a pH near its pKa

NaH2AsO3  (acid) has Ka that I can't find,  it is the Ka2 of H3AsO3 Arsenious acid ( not to be confused with Arsenio Hall)

======================

the ideal buffer of a weak base has a pOH = its pKb

NH3 , K = 1.8e-5, has a pKb = 4.74, so its ideal buffer pOH is 4.74,  a pH of 9.26,  

C6H5NH2, has a Kb = 4.3 e-10 , which has a Kb = 9.37,  so the pOH = 9.37,  and the pH = 4.63

============================

your answers:

[H3O+] = 10-4  would be C6H5NH2/ C6H5NH3Cl

[H3O+] = 10-9  would be     NH3/NH4Cl

[H3O+] = 10-7  should be NaH2AsO3/Na2HAsO3

=============

as an edit, I have found that (H2AsO3)-1 is "virtually undissociated" ,  that's a pH = 7

so the above answers are fine