Its only asking for the mass of the compound P2O5. Basically add the elements' molar mass.
Molar mass of Phosphorus: 30.97 Molar Mass of Oxygen: 16
There is 2 atoms of phosphorus, so you multiply its mass by 2. 30.97 x 2 There is 5 atoms of oxygen, so you multiply its mass by 5. 16 x 5 Then add both masses.
