Answer
There are $1.66\times 10^{27}$ molecules of $H_2O$ in the body.
Work Step by Step
A hydrogen atom consists of 1 proton.
An oxygen atom consists of 8 protons and 8 neutrons.
A molecule of $H_2O$ consists of 10 protons and 8 neutrons.
The mass of one molecule of $H_2O$ is $18~u$
We can find the mass of one $H_2O$ molecule:
$m = (18~u)(1.66\times 10^{-27}~kg/u) = 2.99\times 10^{-26}~kg$
We can find the mass of water in the body:
$m_w = (0.62)(80.2~kg) = 49.724~kg$
We can find the number of $H_2O$ molecules in the body:
$\frac{49.724~kg}{2.99\times 10^{-26}~kg} = 1.66\times 10^{27}$
There are $1.66\times 10^{27}$ molecules of $H_2O$ in the body.
