Answer:
+5
Step-by-step explanation:
The oxidation number of phosphorus can be obtained as follows:
H4P2O7 = 0
4(+1) + 2P + 7(—2) = 0
4 + 2P —14 = 0
Collect like terms
2P = 14 — 4
2P = 10
Divide both side by 2
P = 10/2
P = +5
The oxidation number of phosphorus is +5