Question

_______________ is a monoacidic base whereas _____________ is a diacidic base.
A. $KOH$, $Mg{{(OH)}_{2}}$
B. $KOH$, $Fe{{(OH)}_{3}}$
C. $Mg{{(OH)}_{2}}$, $KOH$
D. None of the above

Answer 1

Hint: Acidity of base is defined as the no of ionisable hydroxide ions ($O{{H}^{-}}$) present in one molecule of a base. We can hence find the acidity of a base by finding out the number of ionizable hydroxide ions it produces.

Complete step by step solution:
The number of ionizable hydroxide ($O{{H}^{-}}$) ions present in one molecule of base is called the acidity of bases. For example $NaOH$ is monoacidic as it furnishes only one $O{{H}^{-}}$ion. On the basis of acidity bases can be classified into three types: monoacidic, diacidic and triacidic.
We can say,
When one molecule of a base on complete ionization produces one hydroxide ion, the base is said to be a monoacidic base.
When one molecule of base on complete ionization produces two hydroxide ions, the base is said to be diacidic. Similarly, when one molecule of base on ionization produces three hydroxide ions, the base is triacidic.
Here we can see, On complete ionization,
$KOH$ produces 1 hydroxide ion and $Mg{{(OH)}_{2}}$ produces 2$O{{H}^{-}}$ ions.
Hence, is a monoacidic base and $Mg{{(OH)}_{2}}$ is a diacidic base.
Thus, Option A would be the correct answer.

NOTE: We must keep in mind Acidic strength and acidity are opposite terms.
Acidic strength represents the ability of an acid to release ${{H}^{+}}$ ions. More the tendency to release${{H}^{+}}$ ions in aqueous solution ,more is the acidic strength. A strong acid will have higher dissociation constant than a weak acid whereas acidity is the no. of $O{{H}^{-}}$ions a base can release from one molecule.
Hence we must not confuse these two terms.