# What is the mass in grams of 1.000 mole of P_2O_5?

The mass in grams of $1.000$ $m o l$, ${P}_{2} {O}_{5}$?
$\text{Mass}$ $=$ (2xx31.00+5xx15.999)*g*mol^-1=??*g*mol^-1. All I have done here is to take the atomic masses and mulitplied them appropriately. From where did I get the atomic masses?
Note that ${P}_{2} {O}_{5}$ is an empirical formula. The actual beast is ${P}_{4} {O}_{10}$, which has a molecular mass of $283.89 \cdot g \cdot m o {l}^{-} 1$.