# What is the mass of a mole of water?

Well, $18 \cdot g \cdot m o {l}^{-} 1$.
Just as $\text{Avogadro's number}$ of hydrogen atoms, ""^1H, has a mass of $1 \cdot g$. $\text{Avocado's number}$ of ""^16O, has a mass of $16 \cdot g$. And $\text{Avocado's number}$ of ""^18O, has a mass of $18 \cdot g$.
We use $\text{Avogadro's number}$ as we would any other collective number, $\text{a dozen}$, $\text{a gross}$, etc. Of course $\text{Avogadro's number}$, $6.022 \times {10}^{23} \cdot m o {l}^{-} 1$ is absurdly large.