# How do I identify an amphoteric compound based on the acid form?

Nov 8, 2016

Just look at it and try adding a proton or taking away a proton. If you generate a species that has a reasonable charge or is neutral, then it probably is a reasonable amphoteric compound.

In general, the conjugate base of a polyprotic acid is a good choice.

${\text{HCO}}_{3}^{-}$ is amphoteric because it can react as either an acid or a base.

This shows it acting as a Bronsted base, since it accepts a proton to become its conjugate acid, carbonic acid, which decomposes into carbon dioxide gas and water liquid:

$\text{HCO"_3^(-)(aq) + "H"^(+)(aq) rightleftharpoons "H"_2"CO"_3(aq) rightleftharpoons "CO"_2(g) + "H"_2"O} \left(l\right)$

This shows it acting as a Bronsted acid, since it donates a proton to become its conjugate base, carbonate:

${\text{HCO"_3^(-)(aq) rightleftharpoons "H"^(+)(aq) + "CO}}_{3}^{2 -} \left(a q\right)$

Some other examples are:

• ${\text{H"_2"PO}}_{4}^{-}$ (dihydrogen phosphate)
• ${\text{HSO}}_{4}^{-}$ (hydrogen sulfate)
• ${\text{H"_2"C"_6"H"_5"O}}_{7}^{-}$ (a form such as monosodium citrate can generate this species in water)

Can you write reactions for these, in a similar manner to the above?