# How do you calculate the osmolarity of seawater?

Sep 29, 2016

#### Answer:

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:

$\text{Ion"color(white)(m)"c/g·L"^"-1"color(white)(m)"c/mmol·L"^"-1}$
stackrel(———————————)("Cl"^"-"color(white)(mll)19.35 color(white)(mmm)545.8)
$\text{Na"^"+} \textcolor{w h i t e}{m} 10.76 \textcolor{w h i t e}{m m m} 468.0$
$\text{SO"_4^"2-} \textcolor{w h i t e}{m l l} 2.71 \textcolor{w h i t e}{m m m l l} 81.2$
${\text{Mg}}^{2 +} \textcolor{w h i t e}{m} 1.29 \textcolor{w h i t e}{m m m l l} 53.1$
${\text{Ca}}^{2 +} \textcolor{w h i t e}{m l} 0.41 \textcolor{w h i t e}{m m m l l} 10$
${\text{K}}^{+} \textcolor{w h i t e}{m m l} 0.40 \textcolor{w h i t e}{m m m l l} 10$
stackrel(———————————)(color(white)(mmll)"TOTAL" = 1168)

Thus, if you consider just the $\text{NaCl}$, 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.