Question

# Ammonia gas dissolves in water to form $N{H_4}OH$. In this reaction water acts as:A.A conjugate baseB.A non-polar solventC.An acidD.A base

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Hint:
Ammonia and water combine to form $N{H_4}OH$. In this reaction, the role of ammonia and water can be explained with the help of the Bronsted-Lowry acid-base theory.

Bronsted–Lowry theory was proposed by Johannes Nicolaus Bronsted and Thomas Martin Lowry which deals with the theory of acids and bases. The theory states that a compound that can transfer a proton to any other compound acts as an acid, and a compound that can accept a proton acts as a base in a reaction. A proton is a positively charged hydrogen atom denoted by ${H^ + }$.
$N{H_3}(g) + {H_2}O(l)\xrightarrow{{}}N{H_4}^ + (l) + O{H^ - }(l)$
Ammonia and water molecules react with each other to give ammonium hydroxide. It is an acid-base reaction. According to Bronsted–Lowry concept ammonia accepts a proton to form $N{H_4}^ +$ ion so it acts as a base and the ammonium ion is the conjugate acid. The water molecule donates a proton to ammonia to generate $O{H^ - }$ ion. Hence water acts as an acid and the $O{H^ - }$is the corresponding conjugate base.
Note: The proton donating power of a compound makes it an acid and a proton accepting power makes it a good base. In other words an acid viz.$HCl$ is an acid because it donates a proton that is accepted by a base that accepts the proton.