Question

Orthophosphoric acid can dimerize on dehydration - True or False?

• A. True
• B. False

Answer 1

The correct option is A True

When phosphoric acid is dehydrated pyrophosphoric acid is produced as one of the products. The structure of pyrophosphoric acid is as shown below. As you can see,$H_4{P}_2{O}_7$ can be construed as two phosphoric acid molecules dimerized by dehydration and removal of only one water molecule.

In fact, molten pyrophosphoric acid quickly establishes an equilibrium with a mixture of phosphoric acid, pyrophosphoric acid and polyphosphoric acids. Polyphosphoric acids are obtained when more than one water molecule is removed via dehydration giving us a chain of phosphoric acid molecules
It is too simplistic to say that:
$H_4{P}_2{O}_7+H_{2}O\rightleftharpoons 2H_{3}P{O}_4$
But the above reaction is achievable for dilute pyrophosphoric acid solutions. So, yes orthophosphoric acid can be dimerized on dehydration (as is evident from the above equilibrium reaction).