Question

Which is the conjugate acid of ammonia?
A. \[N{H_2^{ - }}\]
B. \[O{H^ - }\]
C. \[NH_4^ {+} \]
D. \[{H_3}{O^ + }\]

Answer 1

Hint:
Conjugate acid is a chemical compound formed when an acid donates protons to the base. We can say it is a base having hydrogen ions added to it. Simple trick to find conjugate acid is to add it from the given compound.

Complete step by step solution:
Conjugate acid-base pairs consist of two compounds, having a charge difference of +/- 1 due to the movement of a single proton.
We can understand conjugate acid-base by bronsted definition of acid and bases. According to this theory bronsted acid is any molecule that can donate a proton and bronsted base is any molecule that can accept a proton.
Let us understand this by a reaction.
\[{H_2}O + N{H_3} \rightleftharpoons N{H_4}^ + + O{H^ - }\]
Acid base conjugate acid conjugate base
The conjugate acid of any species, is the original species plus a proton,\[{H^ + }\]. So add a \[{H^ + }\] unit to \[NH3\], and I get \[N{H_4}^ + \] , ammonium ion. This is the conjugate acid of ammonia.
 By the same procedure, if I remove \[{H^ + }\]from any species, I get the conjugate base. So the conjugate base of sulfuric acid, \[{H_2}S{O_4}\] is \[HS{O_4^{ - }}\]
\[N{H_3} + {\text{ }}{H^ + } \rightleftharpoons {\text{ }}N{H_4}^ + \]
Here,\[N{H_3}\] is the conjugate base and \[N{H_4}^ + \] the conjugate acid.

Correct option is C.

Additional Information:
Let us know about some more conjugate acid-base pairs so our concepts will be clearer.

Acid	Base	Conjugate Acid	Conjugate Base
\[HCl{O_2}\]	\[HCl{O_2}\]	\[Cl{O^ - }_2\]	\[{H^{}}_3{O^ + }\]
\[{H_2}O\]	\[Cl{O^ - }\]	\[O{H^ - }\]	\[HClO\]
\[HCl\]	\[{H_2}P{O_4}^ - \]	\[{H_2}P{O_4}^ - \]	\[{H_3}P{O_4}\]

Note:
Just you have to remember the trick is add \[{H^ + }\]to get conjugate acid. Try to write reactions so you will not be confused. You can also use option elimination methods to eradicate wrong options.