What is the name of the compound # P_2O_5#?
Phosphorus (V) oxide, or phosphorus pentoxide
It is the anhydride of phosphoric acid, and has applications as a dessicant and drying agent (amongst other things).
It readily undergoes hydrolysis to reform phosphoric acid.
The name is either phosphorous (V) oxide, the "V" denoting the oxidation state of the phosphorus, which is 5 (more electronegative atom oxygen takes up its standard oxidation number of -2, and phosphorous therefore has an oxidation number of: [0 - (-2 x 5)]/2 = +10 / 2 = 5)
Alternatively phosphorus pentoxide (penta denoting "5 of....") works too.