"what is the oxidation number of phosphorus in h2p3o10? "

             Oxidation Number of Phosphorous  =  + 6

Explanation:

According to Rules of Assigning Oxidation States;

          i)  The Oxidation State of Hydrogen atom in a compound is assigned as +1 except for Hydrides i.e. NaH and MgH₂ where it is assigned with -1.

          ii) The Oxidation State of Oxygen in a compound is assigned as -2 except for Peroxides where it is given -1 and OF₂ where it is assigned with +1 respectively.

Oxidation State of P in H₂P₃O₁₀;

As over all molecule is neutral we can write it as,

                       H₂P₃O₁₀  =  0

Putting oxidation number of H and O,

                       (+1)₂ + P₃ + (-2)₁₀  =  0

                       (+2) + P₃ + (-20)  =  0

                       + 2 + (P)3 -20  =  0

                       (P)3  =  + 20 - 2

                       (P)3  =  + 18

                       P  =   + 18 ÷ 3

                       P  = + 6