Answer
$\approx 4 \times10^{30}J$.
Work Step by Step
Find the mass of water in the oceans. The total mass of water is volume multiplied by density.
Assume an average ocean depth of 5000m, covering 70 percent of the Earth. Use a density of $1000kg/m^3$.
M=0.7(surface area)(depth)(density)
$$ M=0.7(4 \pi (6.38\times10^6m)^2)(5000m)( 1000kg/m^3)=1.79\times10^{21}kg$$
Calculate the number of water molecules by dividing M by water’s molar mass of water, then multiplying by Avogadro’s number.
A water molecule has 2 hydrogen atoms, so we multiply the number of H atoms by the percentage of deuterium to find the number of deuterium atoms.
$$ N=1.377\times10^{43}deuterium\;atoms$$
Consider the reactions of equations 31–8a and 31–8b. Four deuterons produce 7.30 MeV of energy. Find the total amount of energy.
$$ E=(1.377\times10^{43}deuterium\;atoms)\frac{7.30MeV}{4\;deuterons}\frac{1.60\times10^{-13}J}{MeV}$$
$$\approx 4 \times10^{30}J$$