How much water needed to power the world through oxidization of hydrogen

[slapshotphil]

Homework Statement

The annual world use of liquid hydrocarbon fuels for transportation purposes currently generates about 10^17 kJ. If we could instead run our transportation system by oxidizing hydrogen gas to gaseous water, we would likely have to obtain the hydrogen by electrolyzing water. Suppose we made and stored enough hydrogen for one year by electrolyzing seawater. By how much would the level of the oceans decrease (in m)?
ΔH=-245.12KJ/mol

Homework Equations

pv=nrt
stoichiometry
electrolyzing water: 2H₂O --> 2H₂ + O₂

The Attempt at a Solution

I have gotten to the point where i have calculated I need 4.17x10¹⁵ mol of seawater. I am unsure of how to now calculate the decrease in ocean water.

 

[AGNuke]

You might want to search for seawater surface area, that is 71% of the surface area of earth. Consider the Earth spherical with some radius. Convert moles into volume of water and viola, you get your answer.

 

[slapshotphil]

turns out i needed to calculate the volume of seawater by using the molar mass and the density rather than through pv=nrt