Weird question in the Cracking Chemistry of PR

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1
  1. Which of the following species is amphoteric?
    (A) Na3PO4
    (B) HSO4–
    © KOH
    (D) HNO3
    (E) C2O2–4
    The answer is B, which is weird because my textbook said HSO4- is an acid and can never accept H+ to recreate H2SO4. Any thought on this?
2

Obviously that isn’t the case in concentrated solutions of sulfuric acid which exists primarily as the H2SO4 species.

3

@JustOneDad Eh, I don’t get it. What does concentrated solution of H2SO4 have to do with the acidity of ion HSO4-, in water? The half-reactions written in my textbook are:
H2SO4 -> HSO4- + H+ (Ka1 = 10^3)
HSO4- <-> SO42- + H+ (Ka2 = 1.2*10^-2)

4

Hello, this is what I think of the question… First of all an amphoteric species is an acid that can act as an acid or a base. So we can eliminate answer choices A, C, D… So now you have B and C left pretty good chance to get it right…
Now onto the explanation of this…
Let’s take a simple example:
H2O +H2O <–> H3O+ + OH-
This is a VERY important equation, what it says is that water ionizes and produces hydroxide and hydronium ions. These ions give water it’s conductivity. But only a small percent of all the water molecules gets turned to ions actually that is related to kw… Anyway what is important here is the fact that there are 2 water molecules one is giving a H+ (Acid)and the other reviving it(base). So here we can say that the water is amphoteric because it acts as an acid and as a base.
Now for HSO4-…
H2SO4<—> HSO4- + H+ (here sulfuric acid ionizes and loses one H+. Because this acid has more than one H there will be more than one equation each takes away one H until it becomes SO4 2-)
HSO4- <—> SO4 2- +H+

Ok so in the first equation if you notice in the products side the HSO4- is a conjugate base (a conjugate base is an acid that has lost H+ it will act as a base and accept a H+ if the reverse reaction (HSO4- and H+ combine)occurs.)
In the second equation HSO4- is acting as an acid

One it acts as an acid the other base… Amphoteric!!

Hope I helped :slight_smile:

5

@mariam2016 So they were actually asking whether HSO4- is THEORETICALLY amphoteric or not? Since it’s clear all the other answers were wrong, I picked HSO4- even though I was a bit sceptical about its basicity in water. In my defense though, the Ka1 was too big for the reverse reaction to happen. Anyway, thanks for clearing that up.