Answer
See answers.
Work Step by Step
a. Find the volume of the oceans by treating them like a thin spherical shell.
$$V=0.754\pi R^2_{Earth}(depth)=(0.75)(4\pi)(6.38\times10^6m)^2(3000m)=1.15\times10^{18}m^3$$
Assume that seawater’s density is $1025\frac{kg}{m^3}$.
$$(1.15\times10^{18}m^3) 1025\frac{kg}{m^3}(\frac{mol}{0.018kg})\approx 7\times10^{22}moles$$
b. Now approximate the number of water molecules.
$$(6.55\times10^{22}moles) \frac{6.02\times10^{23}molecules}{mole}\approx4\times10^{46}molecules$$