# How do you calculate molecular weight?

Typically we calculate a molar mass, i.e. a mass of a given quantity of molecules. The given quantity is ${N}_{A}$, $\text{Avogadro's number}$ $=$ $6.022 \times {10}^{23} \cdot m o {l}^{-} 1$.
$\text{Avogadro's number}$ of ""^1H has a mass of $1.00 \cdot g$ precisely; $\text{Avogadro's number}$ of ""^12C has a mass of $12.00 \cdot g$ precisely. If I have $\text{Avogadro's number}$, $6.022 \times {10}^{23}$, individual ""^12C^1H_4 molecules, it follows that I have $\left(4 \times 1.00 + 12.00\right) \cdot g$. Typically we would say methane has a molar mass of $16.00 \cdot g \cdot m o {l}^{-} 1$