Question

The conjugate base of \[HP{O_4}^{2 - }\] is
A. $P{O_4}^{3 - }$
B. ${H_2}P{O^ - }_4$
C. ${H_3}P{O_4}$
D. ${H_3}P{O_3}$

Answer 1

Hint: The strength of an acid and its conjugate base are inversely related. We can define conjugate base as a substance formed when an acid loses a hydrogen ion. The substance is base as it donates a hydrogen ion from acid. According to Bronsted Lowry theory of acids and bases, an acid is a proton donor and base is a proton acceptor. The general reaction between a conjugate acid and conjugate base is as follows; $HX + {H_2}O \to {X^ - } + {H_3}{O^ + }$. Conjugate can be easily recognised in an acid-base reaction as it is an anion.

Complete answer:
In the given problem the conjugate base of \[HP{O_4}^{2 - }\]can be recognised with the help of its reaction with water which is : \[HP{O_4}^{2 - } + {H_2}O \to P{O_4}^{3 - } + {H_3}{O^ + }\] hence in this reaction we observed that \[HP{O_4}^{2 - }\] removes one hydrogen ion and gives $P{O_4}^{3 - }$ which is conjugate base. The conjugate base of \[HP{O_4}^{2 - }\] is $P{O_4}^{3 - }$.Thus option A is the correct answer to this problem. Some acids are capable of donating multiple protons hence the conjugate base of an acid can be acidic itself.

Note: We know that all acids have conjugate bases. In reaction, $HX + {H_2}O \to {X^ - } + {H_3}{O^ + }$ ;$HX$ reacts with water molecules to create conjugate acid base pairs. Here ${X^ - }$ is conjugate base. An acid donates a proton from its conjugate base while base can accept a proton from its conjugate acid. The conjugate acid and base pairs make a simple statement about relative statements about relative strength of acid and bases. On the other hand it has observed that a conjugate base is left over after acid donates a proton during chemical reaction.