+5 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

Oxidation numbers:Ca +2PxO -2 +2*3 +x*2 – 2*6 = 06 + 2x -12 = 02x = 6x = +3 Oxidation number of P is +3.