Question

How do you calculate the osmolarity of seawater?

Answer 1

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.