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..?

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.

To determine whether a pair of substances can form a buffer solution, we need to check if they contain a weak acid and its conjugate base, or a weak base and its conjugate acid.

In option A, NaC2H3O2 and HCl do not form a buffer solution because they do not contain a weak acid and its conjugate base. NaC2H3O2 is the conjugate base of the weak acid HC2H3O2, but HCl is a strong acid.

In option B, RbOH and HBr also do not form a buffer solution because they do not contain a weak base and its conjugate acid. RbOH is a strong base, and HBr is a strong acid.

In option C, NH3 and NH4Cl can form a buffer solution. NH3 is a weak base, and NH4Cl is the conjugate acid of NH3, making it a weak acid.

In option D, H3PO4 and KH2PO4 cannot form a buffer solution because they do not contain a weak acid and its conjugate base. H3PO4 is a strong acid, and KH2PO4 is a salt, not forming a conjugate base.

In option E, KOH and HF can form a buffer solution. KOH is a strong base, and HF is a weak acid.

Therefore, the correct answer is option B, RbOH and HBr, as they do not form a buffer solution since they do not contain a weak base and its conjugate acid.

To determine which pair cannot be mixed together to form a buffer solution, we need to understand what a buffer solution is. A buffer solution is a solution that resists changes in pH when an acid or a base is added to it. Buffer solutions are typically composed of a weak acid and its conjugate base or a weak base and its conjugate acid.

When considering each given pair, we need to check if they contain a weak acid and its conjugate base or a weak base and its conjugate acid. If they do, then they can potentially form a buffer solution. If they do not, then they cannot form a buffer solution.

In the case of pair B) RbOH and HBr, we need to determine if RbOH is a weak base and HBr is its conjugate acid or HBr is a weak acid and RbOH is its conjugate base.

To do this, we can compare the acidity/basicity of the compounds. HBr is a strong acid, meaning it dissociates completely in water, and therefore, it cannot be classified as a weak acid. Similarly, RbOH is a strong base, meaning it also dissociates completely in water and cannot be considered as a weak base.

Since neither compound in pair B) is a weak acid or weak base, they cannot form a buffer solution. Therefore, the answer is B) RbOH, HBr.

It is worth noting that NaC2H3O2 and HCl (pair A) can form a buffer solution because NaC2H3O2 is the conjugate base of acetic acid (a weak acid) and HCl is a strong acid. The other pairs (C), (D), and (E) are also capable of forming buffer solutions because they involve conjugate acid-base pairs.