Question

Why does phosphoric acid react with three times more than sodium hydroxide, when the solutions are mixed to form the salt?
a.) It is highly soluble
b.) It has 3 hydrogen to be replaced
c.) It is insoluble
d.) Both B and C

Answer 1

The reaction of phosphoric acid with sodium hydroxide is a neutralization reaction. Upon the neutralization reaction the acid will lose its protons and form salt and water.

Complete step by step answer:
Neutralization reaction is a reaction in which an acid and a base react with each other and form salt and water.
Here we have phosphoric acid and sodium hydroxide. Let’s understand the neutralization reaction of phosphoric acid and sodium hydroxide.
Phosphoric acid has three removable protons, there is stepwise removal of each of the proton:
${{H}_{3}}P{{O}_{4}}+NaOH\to Na{{H}_{2}}P{{O}_{4}}+{{H}_{2}}O$

When there is the removal of the first proton the product formed is monobasic sodium phosphate. Similarly the neutralization reaction for the removal of second proton is:
$Na{{H}_{2}}P{{O}_{4}}+NaOH\to N{{a}_{2}}HP{{O}_{4}}+{{H}_{2}}O$

The product formed is dibasic sodium phosphate. And neutralization reaction for the removal of third proton is:
$N{{a}_{2}}HP{{O}_{4}}+NaOH\to N{{a}_{3}}P{{O}_{4}}+{{H}_{2}}O$
Here the product formed is tribasic sodium phosphate.

Hence we can say that the phosphoric acid produces three protons per acid molecule. So there are three hydrogen atoms that have to be replaced by the metal ions. The overall reaction is:
${{H}_{3}}P{{O}_{4}}+3NaOH\to N{{a}_{3}}P{{O}_{4}}+3{{H}_{2}}O$
So, the correct answer is “Option B”.

Note: At high temperature the molecules of phosphoric acid react together and combine to form dimer, trimmer and even the long polymeric chains. Pure phosphoric acid is a white crystal like solid and has a melting point of 42.35$^{0}C$.