Question

A 15.5 gram sample of diphosphorous pentoxide contains how many grams of phosphorous?

Answer 1

formula of diphosphorous pentoxide = P₂O₅  

lets see the atomic masses of each

Atomic mass of P = 30.97 g/mol

Atomic mass of O = 16.00 g/mol

now lets see the total mass of P₂O₅

Mass of P₂O₅ = 2 $\times$ 30.97 g/mol + 5 $\times$ 16 g/mol = 141.9 g

Mass of P in P₂O₅ = 2 $\times$ 30.97 = 61.9 g

fraction of P in P₂O₅ = 61.9 /141.9

So 15.5 gram of samples contains P = 15.5 g $\times$ 61.9 /141.9 = 6.76 g P

So our answer is = 6.76 g P