Question

Conjugate base of ${H_2}PO_4^ - $ is:
(A). ${H_3}P{O_4}$
(B). ${P_2}{O_5}$
(C). $PO_4^{3 - }$
(D). $HPO_4^{2 - }$

Answer 1

Hint: When a base accepts a proton, it leads to the formation of conjugate acid and when an acid loses a proton, the left over substance is known as conjugate base.

Complete step by step answer: The given compound ${H_2}P{O_4}^ - $ is known as dihydrogen phosphate ion and has a molecular mass of 97 g. This compound or ion shows the presence of two OH bonds. Most of the dihydrogen phosphate salts are colorless, water soluble and non-toxic.
Now let us understand the bronsted-lowry concept. According to the bronsted-lowry concept, the relative strength of two acids are compared by comparing their tendency to donate protons. Similarly the relative strength of two bases are compared by comparing their tendencies to accept protons. The substance formed after the acid loses proton (${H^ + }$) is known as conjugate base and the substance formed after a base accepts a proton (${H^ + }$) is known as conjugate acid.
So we got to know that the conjugate base is formed when we remove ${H^ + }$from a compound. When a proton is removed, to balance the electrical neutrality, a negative charge appears on the remaining substance. So the conjugate base of ${H_2}P{O_4}^ - $ is $HP{O_4}^{2 - }$.
The compound $HP{O_4}^{2 - }$ is known as monohydrogen phosphate.
${H_2}P{O_4}^ - \to HP{O_4}^{2 - } + {H^ + }$
The option (D) is the correct answer.



Note: A conjugate base is formed from an acid and conjugate acid is formed from a base. Stronger the acid or base, weaker will be their conjugate base or conjugate acid respectively.