Henry's law has some to do with finding the concentration of a gas in water based on the amount of the gas bouncing out of the water i.e. gas concentration above the water.

The math for it is super complicated.

But In my wiki search I thought I found the constant 0.8317 is good enough for CO2, assuming normal water density (not salt water etc) and temp of 70F.

http://en.wikipedia.org/wiki/Henry's_law-------------------------------------------------------

ppm is parts per million or

ppm(volume) = mg/L

ppm(weight) = molar weight/1000 kg/m^3

1000 kg/m^3 is the density of water and can be substituted with 1000000 mg/L to get the formula simplified.

for my thought to get the weight of it would be

co2_weight = co2_volume/1000000 * .8317 (henry's) * 44.01 (mole weight of CO2) / 1000000