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What is the molarity of salt in seawater?

What is the salt molarity concentration for Pacific ocean waters?
L. Huang

• Sea water contains a lot of different dissolved salts. Do you want the concentration of the NaCl in sea water, or the total salt concentration as NaCl?
• What part of the Pacific Ocean? As you can see from this map of Pacific ocean salinities [NOAA], the concentration varies with location. (Click the image for a full size version).

Once you've decided whether you want actual NaCl molarity or total salt as NaCl molarity, and once you've picked a spot on the map above, you can convert units to obtain a molarity of NaCl from a salinity in PSS. Suppose your spot has a salinity of 35 PSS. Then your calculation will look like

PSS g sea salt/kg seawater g Cl/kg seawater
mol Cl/kg seawater mol NaCl/kg seawater mol NaCl/L seawater

The PSS stands for "Practical Salinity Scale". It's defined in terms of electrical conductivity measurements but for your purposes, you can probably use the following approximate conversion factors:

1 PSS unit = 1 g sea salt/ kg seawater
1.80655 g sea salt = 1 g Cl

The last step converts kg seawater to L seawater, and you'll need the density of seawater for that. It's between 1.02 g/mL and 1.07 g/mL, but if you want precision you'll have to look up the number in a physical oceanography text. (I can't give you a number because the density will depend on the salinity you use- and on the ocean temperature and pressure).

References

1. NOAA Map The salinity map was obtained from the National Oceanic and Atmospheric Administration Climate Server at http://teal.wrc.noaa.gov/tmap-bin/climate_server (now apparently offline).

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! What is the molarity of salt in seawater?