# Homework Help: I think I'm making this problem way too complicated than it really is.

1. Aug 23, 2006

### bearhug

Assuming that 71% of the Earthâ€™s surface is covered with water at an average depth of 2.1 mi, estimate the mass of the water on the Earth in kilograms.

What I originally did for this problem was find the total surface area of the earth to be 1.97e8 mi^2 and multiplied by 0.71 to get the water's surface area to be 1.40e8mi^2. Then I multiplied that to 2.1 mi to get volume and found mass using D= m/V and converting to kg. The answer I got was 1.22e15 kg and it's wrong. (This is done on a computer program). I'm not sure if my math is wrong or if I'm making it alot more complicated than it really is.

Any help would be appreciated

2. Aug 23, 2006

### Staff: Mentor

Looks like the correct approach to me. What did you use to get the surface area of the earth? You are mixing units in your explanation of your work -- did you use the correct conversion factors? Maybe post a more complete listing of your work, and we can see if anything pops out at us.

EDIT -- not really "mixing" units...I misspoke. Maybe it's an error in the mass/Volume part?

3. Aug 23, 2006

### chroot

Staff Emeritus
You seem to have the correct approach, but perhaps you made an arithmetic or unit conversion error. I get this:

Surface area of Earth: 5.1 × 1014 m2
Volume of oceans: 1.2 × 1018 m3
Mass of oceans: 1.2 × 1021 kilograms

- Warren

Last edited: Aug 23, 2006
4. Aug 23, 2006

### bearhug

After looking at all replies I checked my math again and got what chroot got, 1.2x10^21 kg thanks for the help, at least I know that I understand the problem.