Question

Conjugate Base of $HPO_4^{2 - }$ is:
A) $PO_4^{3 - }$
B) ${H_2}PO_4^ -$
C) ${H_3}P{O_4}$
D) ${H_4}P{O_3}$

Answer 1

According to Bronsted Lowry Theory conjugate base is a chemical compound which is formed when an acid donates a proton to base or when base gains a hydrogen ion. For example, the conjugate base of hydrochloric acid $HCl$ is chloride ion $C{l^ - }$ .

Complete answer:
Let us first look at the definition of conjugate base.
According to Bronsted Lowry theory conjugate acid is a chemical compound which is formed when an acid donates a proton to the base. Conjugate base is a substance which is formed when acid loses hydrogen ion.
Let us take an example:
Reaction used : Acid + Base $\rightleftharpoons$ Conjugate Base + Conjugate Acid
So in this question we are going to find out the conjugate base of $HPO_4^{2 - }$.
So according to the above knowledge the conjugate base of $HPO_4^{2 - }$ is $PO_4^{3 -}$.
Reaction involved is :
HPO_4^{2 - }$$$ \to $ $PO_4^{3 - }$ + ${H^ + }$ In this case HPO_4^{2 - H}$$$ loses hydrogen and forms $PO_4^{3 - }$ or $H^+$ in the solution. According to our definition of conjugate acid base pair, Conjugate base is a substance which donates a proton so here proton is donated by $PO_4^{3 - }$. Therefore $PO_4^{3 - }$ is the conjugate base of $HPO_4^{2 - }$.

Hence the answer to the above question is $PO_4^{3 - }$ which is option ( A ).

Note: Always keep in mind that conjugate acid is formed by gaining hydrogen ion and conjugate base is formed by losing hydrogen ion. Bronsted Lowry Theory gave the concept of Conjugate Acid Base pair