[Fiona]

Fiona  asked the Naked Scientists:
   
Hi Chris, really love your site.  Need some help with a coefficient for:
 
In water (ie a lake), how much O2 is needed to lift 1 Kg of Mass?
 
Thanks for your assistance.
Best Regards
Fiona

What do you think?

[RD]

If by "O2" you mean air, it's mostly N₂ ...



http://en.wikipedia.org/wiki/Air#Composition

Further reading ...

http://en.wikipedia.org/wiki/Air#Density_and_mass

http://en.wikipedia.org/wiki/Boyancy

[Phractality]

To float, the 1 kg mass plus the mass of O₂must displace an equal or greater mass of water. So the mass of O₂depends on the density and depth of the water and the density of the 1 kg mass. If you have a balloon full of O₂which is just big enough to make the 1 kg mass float at the surface, and you then pull it below the surface, the balloon will shrink as the pressure increases. Below a certain depth, it will sink.

[JP]

You need no O₂ to make 1 kg of ice float. 

(Which is due to it's density being lower than water, as others have pointed out.)

[Geezer]

I don't think we have enough information to answer the question. As Phrac points out, the lift is a function of the depth, so, if you want to lift something from the bottom of the lake, you'll need to know how deep the lake is.