How Do You Calculate the Mass of All the Water on Earth?

[Brewer]

If I've been given the density of water, how do I go about calculating the mass of all the water on Earth.

Of course, density = m/v, and whilst water covers 2/3 (?) of the Earths surface, how can I work this into the volume? Would it be the 2/3 * surface area of Earth * average depth of the oceans?

This is assuming that sea water has the same density as fresh water (which is obviously not the case, but still - its what the question states)

 

[vsage]

That equation is a good approximation if the surface area of the Earth around the depth you are told is comparable to the surface area of the surface of the earth. I'm a little unsure what you're actually given in the question and what you're not, though.