# How many molecules are in a sample of water with a mass of 44.99 grams?

$\frac{44.99 \cdot g}{18.01 \cdot g \cdot m o {l}^{-} 1}$ $\times$ ${N}_{A}$, where ${N}_{A}$ $=$ $\text{Avogadro's number } 6.022 \times {10}^{23} \cdot m o {l}^{-} 1$.
By definition $1$ mole of water has a mass of $18.01 \cdot g$. And in such a mass there are ${N}_{A}$ water molecules.