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How much salt water is in the ocean, and how much fresh water is there?
Question Date: 2013-10-07
Answer 1:

Interesting question! There are parts of the ocean that are saltier than others, for example, surface waters near the equator where evaporation removes water and concentrated salt. There are parts that have more freshwater as well. But, in general, the fresh and salt water in the ocean are very well mixed, so it’s hard to say exactly how much fresh water and how much salt water there is…

Even though it is a mixture, one way to address your question is to figure out how much fresh water you would need to add to water that is saturated in salt to end up with the composition of the ocean. The open ocean has salt concentration of about 30 grams per kilogram (Hamblin & Christiansen, 2004); keep in mind that the concentration of salt is different in different parts of the ocean. I found varying information about the concentration of water that is saturated in salt (saturated means that no more salt can go into solution). The solubility of salt in water depends on many factors (primarily temperature and pressure), but let’s use a reasonable value of ~ 350 grams per kilogram. We can see immediately that the concentration of salt in ocean water (30 g/kg) is nowhere near saturated.

Let’s due a quick mixing calculation:

concmix = (fsat.)x(concsat) + (ffresh)x(concfresh)

concmix= concentration of mixture (ocean water) = 30 g/kg
concsat= concentration of saturated water= 350 g/kg
concfresh= concentration of fresh water= 0 g/kg
fsat= fraction of saturated water
ffresh= fraction of fresh water
ffresh= 1¬–fsat (we know this because the fractions must add up to one)

plugging numbers in and substituting (1–fsat) for ffresh:
30 g/kg= (fsat)x(350 g/kg) + (1–fsat)x(0 g/kg)
solving for fsat :

fsat = (30 g/kg)/(350 g/kg) = 0.086
and ffresh :

ffresh = 1– 0.086 = 0.914

I know that looks like a long calculation, but it is simply an equation for how much fresh water you would need to mix with salt-saturated water to end of with the composition of ocean water. The answer is: ocean water is 8.6% salt-saturated water and 91.4% fresh water.

Answer 2:

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