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)
where:
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.
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