# How do you find formula mass?

Jun 28, 2016

Consider the formula, and sum up the masses of each element in the formula to give the formula mass. You get an answer in $g \cdot m o {l}^{-} 1 , \text{a mass in gram per Avogadro's number of atoms}$.
So let's take an example: $O s {O}_{4}$ (I don't know why I chose that one!). In $1$ $m o l$ of this stuff, there are $1$ $m o l$ metal atoms, and $4$ $m o l$ of $O$ atoms. I take the masses from the Periodic Table, and I get a mass of $\left\{190.23 \left(O s\right) + 4 \times 15.999 \left(O\right)\right\} \cdot g \cdot m o {l}^{-} 1 = 254.23 \cdot g \cdot m o {l}^{-} 1$.