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APChem

Hi I tried to understand this but I don't know why the answer is B:
Which one of the following pairs cannot be mixed together to form a buffer solution:
A) NaC2H3O2, HCl
B)RbOH, HBr
C) NH3, NH4Cl
D) H3PO4, KH2PO4
E) KOH, HF

The answer is B but could you explain why?
I know its not C because they are conj. acid base pairs..but NaC2H4O2 and HCl don't have a common ion...so I don't understand..?

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  1. You are right about C. A can use the C2H3O2 ion to form HC2H3O2 (acetic acid) and now you have the weak acid and its salt.
    B is RbOH and HBr. RbOH is a strong base (think NaOH). HBr is a strong acid (think HCl). That's the main reason B can't be a buffer. D is a buffer because H2PO4^- is the conjugate base for H3PO4.
    E is an acid/base reaction. KOH + HF ==> KF + H2O. But if you have the proportions right you can have some HF left over (not neutralized so you have a weak acid and the salt (KF) and that is a buffer.

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