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 rightarrow 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

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?