# Which of the following can act both as a Bronsted acid and a Bronsted base?

(a) $O^{2-}$

(b) $HCl$

(c) $HSO^-_4$

(d) $Na_2CO_3$

Toolbox:
• Acid is a substance which is capable of donating a proton.
• Base is a substance which is capable of accepting a proton.
Answer: $HSO_4^-$

$O^{2-} \overset{-H^+}{\longleftarrow} OH^- \therefore$ Bronsted Base

$Cl^- \overset{-H^+}{\longleftarrow} HCl \therefore$ Bronsted Acid

$SO_4^{-2} \overset{-H^+}{\longleftarrow} HSO^-_4 \overset{-H^+}{\longleftarrow} H_2SO_4 \therefore$ Both Bronsted Base and Bronsted Acid

$Na_2CO_3 \overset{-H^+}{\longleftarrow} NaHCO_3 \therefore$ Bronsted Base

A Bronsted-Lowry acis is any species that is capable of donating a proton $H^+$
A Bronsted-Lowry base is any species that is capable of accepting a proton which requires a lone pair of electrons to bond to the $H^+$
An Bronsted acids are also acids according to Arrhenius concept. But all Bronsted bases may not be Arrhenius bases. $H_2PO_4^-, HCO_3^-,HSO_4^-$ etc. are Bronsted acids as well as Bronsted bases but according to Arrhenius concept, these are only acids