How do you calculate the osmolarity of seawater?

1 Answer
Ernest Z. · Kate M.
Sep 29, 2016

You add up the molarities of the constituent ions.

Explanation:

The composition of seawater varies around the world, but the concentrations of the six most abundant ions in a "typical" sample are:

#"Ion"color(white)(m)"c/g·L"^"-1"color(white)(m)"c/mmol·L"^"-1"#
#stackrel(———————————)("Cl"^"-"color(white)(mll)19.35 color(white)(mmm)545.8)#
#"Na"^"+"color(white)(m)10.76color(white)(mmm)468.0#
#"SO"_4^"2-"color(white)(mll)2.71 color(white)(mmmll)81.2#
#"Mg"^(2+)color(white)(m)1.29color(white)(mmmll)53.1#
#"Ca"^(2+)color(white)(ml)0.41 color(white)(mmmll)10#
#"K"^+color(white)(mml)0.40color(white)(mmmll)10#
#stackrel(———————————)(color(white)(mmll)"TOTAL" = 1168)#

Thus, if you consider just the #"NaCl"#, the osmolarity of seawater is about 1000 mOsmol/L.

If you include the magnesium, calcium, potassium, and sulfate ions, the osmolarity is closer to 1200 mOsmol/L.

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