# What is the mass of 2.14 * 10^10 molecules of water?

$\approx 6.39 \times {10}^{-} 13 g$
molar mass of water = $18 g m o {l}^{-} 1$
So $6.022 \times {10}^{23}$molecules have mass 18g
mass of $2.14 \times {10}^{10}$ molecules will have mass
$\left(2.14 \times {10}^{10}\right) \times \frac{18 g \cancel{m o {l}^{-} 1}}{6.022 \times {10}^{23} \cancel{m o {l}^{-} 1}} \approx 6.39 \times {10}^{-} 13 g$