Question

The number of water molecules needed to convert one molecule of ${{\text{P}}_{\text{2}}}{{\text{O}}_{\text{5}}}$into orthophosphoric acid is ______________.
A.2
B.3
C.4
D.5

Answer 1

The ortho phosphoric acid has 3 hydroxy groups and one oxo group whereas ${{\text{P}}_{\text{2}}}{{\text{O}}_{\text{5}}}$has 4 oxo groups.

Complete step by step answer:
Ortho phosphoric acid has the formula ${{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}$containing 3 hydroxy groups and one oxo group.
The phosphorus pentoxide,${{\text{P}}_{\text{2}}}{{\text{O}}_{\text{5}}}$can be converted into ortho phosphoric acid by using 3 molecules of water.
${P_2}{O_5} + 3{H_2}O \to 2{H_3}P{O_4}$
This can be shown in detail by following steps showing how each molecule of water gets used to convert phosphorus pentoxide to ortho phosphoric acid. Phosphorous pentoxide,${{\text{P}}_{\text{2}}}{{\text{O}}_{\text{5}}}$, reacts with one molecule of water and gets converted to meta Phosphoric acid (${{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{3}}}$).
${P_2}{O_5} + {H_2}O \to 2HP{O_3}$
The meta Phosphoric acid (${{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{3}}}$) formed reacts with another molecule of water and pyro phosphoric acid(${{\text{H}}_{\text{4}}}{{\text{P}}_{\text{2}}}{{\text{O}}_{\text{7}}}$).
$2HP{O_3} + {H_2}O \to {H_4}{P_2}{O_7}$
The pyro phosphoric acid(${{\text{H}}_{\text{4}}}{{\text{P}}_{\text{2}}}{{\text{O}}_{\text{7}}}$) formed reacts with another molecule of water and we finally get the ortho phosphoric acid,${{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}$.
${H_4}{P_2}{O_7} + {H_2}O \to {H_3}P{O_3}$
Therefore, the number of water molecules needed to convert one molecule of${{\text{P}}_{\text{2}}}{{\text{O}}_{\text{5}}}$into orthophosphoric acid is 3.
So, the correct option is B.

Additional Information : Phosphorous pentoxide,${{\text{P}}_{\text{2}}}{{\text{O}}_{\text{5}}}$is a good dehydrating agent that can remove water molecule under suitable conditions. Ortho phosphoric acid,${{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}$ is a mineral acid that can remove rust and also used in processed foods.

Note:
The number of water molecules in a compound depends on the hydroxyl group present in the desired molecule. The hydroxyl group will easily abstract protons from water and molecules can be converted with ease.