Practice question · Match the pairs

Match each acid to its conjugate base.

$\mathrm{HCl}$
$\mathrm{H_2O}$
$\mathrm{NH_4^+}$
$\mathrm{H_2CO_3}$

$\mathrm{Cl^-}$
$\mathrm{HCO_3^-}$
$\mathrm{NH_3}$
$\mathrm{OH^-}$

Hints

A conjugate base is what remains after the acid donates one proton.
Remove exactly one hydrogen ion, reducing the charge by one.

Show the answer

$\mathrm{HCl}$ → $\mathrm{Cl^-}$
$\mathrm{H_2O}$ → $\mathrm{OH^-}$
$\mathrm{NH_4^+}$ → $\mathrm{NH_3}$
$\mathrm{H_2CO_3}$ → $\mathrm{HCO_3^-}$

Why

Each conjugate base is the acid minus one proton, so the charge falls by exactly one. Note that water appears as an acid here, it is amphoteric and takes either role depending on its partner.

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