Question

One molecule of water $\left(H_{2}O\right)$ contains two atoms of hydrogen and one atom of oxygen. A hydrogen atom has a mass of $1.0u$ and an atom of oxygen has a mass of $16u$, approximately.
How many molecules of water are in the world's oceans, which have an estimated total mass of $1.4\times {10}^{21}kg$?

Answer 1

As we know,

We divide the total mass by mass of each molecule and obtain the (approximate) number of water molecule:

$N\approx \frac{1.4\times {10}^{21}}{3.0\times {10}^{-26}}\approx 5\times {10}^{46}$