Question

State why the basicity of acetic acid is one and acidity of calcium hydroxide is two?

Answer 1

Basicity of an acid is the number of hydrogen ions which can be produced from one molecule of the acids on complete dissociation. Acidity of a base is the number of hydroxyl ions which can be produced from one molecule of the base on complete dissociation.

Complete step by step answer:
Here, we have given acetic acid and calcium hydroxide
Let us write the dissociation equation of acetic acid,
$\Rightarrow C{{H}_{3}}COOH\rightleftharpoons C{{H}_{3}}CO{{O}^{-}}+{{H}^{+}}$
Here, we can see that one molecule of acetic acid on complete dissociation gives one H+. Hence acetic acid monobasic, i.e. the basicity of acetic acid is one.
Let us write the dissociation equation of the calcium hydroxide,
$\Rightarrow Ca{{(OH)}_{2}}\rightleftharpoons C{{a}^{2+}}+2O{{H}^{-}}$
Here, we can see that one molecule of calcium hydroxide gives two hydroxyl ions. So calcium hydroxide is di-acidic, i.e. the acidity of calcium hydroxide is two.

Note:
Also, we can predict the strength of acids/bases by using basicity and acidity respectively. For an acid, if the basicity is more the strength is more and vice-versa. Also for a base, if the acidity is more the strength is more and vice-versa. Remember that acidity and basicity are based on the same chemical reaction, just looking at it from opposite sides, so they are opposites. When evaluating acidity / basicity, look at the atom bearing the proton / electron pair first. Then you may also need to consider resonance, inductive (remote electronegativity effects), the orbitals involved and the charge on that atom.