## College Physics (4th Edition)

There are $1.66\times 10^{27}$ molecules of $H_2O$ in the body.
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.