Question

Which molecule will act as both bronsted acid and base?

• A. $H_{2}O$
• B. $C{H}_4$
• C. $B{F}_3$
• D. $NaOH$

Answer 1

The correct option is A $H_{2}O$

Bronsted acid is a species that is capable of accepting a proton and bronsted base is a species that donates a proton.
Here $H_{2}O$ will act as both bronsted acid and base.
$H_{2}O+N{H}_3\rightleftharpoons O{H}^{-}+N{H}_4^{+}$
Here, water donates its $H^{+}$ to $N{H}_3$ and act as a bronsted acid
$H_{2}O+H_{2}S\rightleftharpoons H_3{O}^{+}+H{S}^{-}$
Here , water accept the $H^{+}$ ions by its lone pair and act as bronsted base.
Hence it amphoteric in nature.
NaOH will act only as bronsted base since the $O{H}^{-}$ ion abstract $H^{+}$ ion
$B{F}_3$ does not act as bronsted acid or base since it electron deficient to abstract $H^{+}$ and it does not have hydrogen to donate.
$C{H}_4$ does not donate its hydrogen because their bonds are covalent in nature and they form precise stable octet structure.