Question

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

Answer 1

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.