Uses It is most effective drying or dehydrating agent below 100oC
Structure of phosphorus pentoxide
3. Oxyacids of phosphorus Phosphorus forms a number of oxyacids. Common oxyacids are given below. Name of Oxyacid
Formula
Basicity
Oxidation state of P
Hydrophosphorus acid
H3PO2
1
+1
Phosphorus acid
H3PO3
2
+3
Orthophosphoric acid
H3PO4
3
+5
Metaphosphoric acid
HPO3
1
+5
Hypophosphoric acid
H2P 2O 6
4
+4
Pyrophosphoric acid
H4P 2O 7
4
+5
(i) Phosphorus acid, H3PO3, Prepartion (i) It is obtained by dissolving phosphorus trioxide in water. P4O6 + 6H2O → H3PO3
(ii) It is also obtained by hydrolysis of phosphorus trichloride. PCl3 + 3H2O → H3PO3 + 3HCl Chlorine is passed over molten white phosphorus under water when phosphorus trichloride formed undergoes hydrolysis. 2P + 3Cl2 → 2PCl3 The solution is heated until the temperature becomes 180oC. On cooling crystals of phosphorus acid are obtained. Properties (a) It is colourless crystalline compound. It melts at 73oC. It is highly soluble in water.
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(h) Mixture of P2O5 and O2 in vapour state combine in presence of electric discharge to form P2O6 called phosphorus peroxide. P2O5 + 1/2O2 → P2O6