Question

The conjugate base of $H_{2}P{O_4}^{-}$ is:
(A) $HP{O_4}^{2-}$
(B) $P_2{O}_5$
(C) $H_{3}P{O}_4$
(D) $P{O_4}^{3-}$

Answer 1

Hint: This question requires the knowledge of different definitions of acids and bases. Acids and bases are classified by Bronsted-Lowry theory, Arrhenius theory and Lewis theory. Conjugate bases are the compounds formed after deprotonation.

Complete Step by step solution
Arrhenius theory:
This theory was given by Swedish scientist Svante Arrhenius in 1887. According to Arrhenius theory, acids are the substances that dissociate in water to yield electrically charged atoms or molecules called ions. These ions include hydrogen ions. Bases are the substances that ionize water to yield hydroxide ions. Hydrogen ions released by the acids cannot stay alone hence it combines with water molecules. After combining it forms hydronium ion. The acidic behavior of many well-known acids like sulphuric acid, hydrochloric acid nitric acid and basic properties of hydroxides like sodium hydroxide, potassium Hydroxide and calcium hydroxide can be explained with the help of Arrhenius theory.
Lewis acid-base theory:
According to Lewis acid base theory, Lewis acid is a chemical species that contains an empty orbital and it is capable of accepting an electron pair. Lewis base is a species which donate a pair of electrons. Lewis acid and Lewis base react and form dative bonds and then they form Lewis adduct. Lewis acid base theory was given by Gilbert Lewis.
Bronsted-Lowry theory:
The Bronsted Lowry theory was given by Johannes Nicholas Bronsted and Thomas Martin Lowry in 1923. According to this theory acid and base react with each other and acid forms conjugate base and base forms conjugate acid. This theory also states that acid is something which donates protons and base is something which accepts protons. In turn, the conjugate base is able to gain or absorb protons in a chemical reaction. And conjugate acid is able to donate protons in chemical reactions. In order to get the conjugate base of any chemical species we will consider the species acid and it will donate hydrogen ions. The conjugate base of $H_{2}P{O_4}^{-}$ is $HP{O_4}^{2-}$ .
Hence the correct answer is option A.

Note
Phosphoric acid is a triprotic acid. It donates 3 hydrogen atoms. Phosphoric acid is also known as orthophosphoric acid. It is used to make phosphate salts for fertilizers. It is also used in dental cements. The use of conjugate bases is also prominent in organic chemistry. An acid is said to be strong if and only if the conjugate base is stable.