So to get the answer use the equation: moles = mass/ RAM the RAM is the relative atomic mass and can be calculated by using the bottom number in the box of the element on the periodic table so for this question the mass of P4 is 161g and the RAM of P4 is 124 (31 X 4) so 161/124 = 1.3 and because you need to work out P2O5 and not P4 2 is half of 4 so you half the moles so the answer should be 0.65
