# What is formula mass and how is it calculated?

The units of formula mass are thus $g \cdot m o {l}^{-} 1$, or, more fundamentally, $\text{amu "=" atomic mass units}$. As chemists, we typically deal with molar quantities, so units of $g \cdot m o {l}^{-} 1$ are appropriate.
So let's try to calclate the formula mass of $N {a}_{2} S {O}_{4}$ in $g \cdot m o {l}^{-} 1$.
$\text{Formula mass } N {a}_{2} S {O}_{4}$ $=$ (2xx22.99+32.06+4xx15.999)*g*mol^-1=??g*mol^-1. From where did I get those numbers?