Question

How do you calculate the osmolarity of seawater?

Answer 1

Osmolarity of a solution is related to the molarity of the constituent ions. Molarity is a unit of concentration. It is defined as the amount of moles of solute dissolved per liter of a solvent.

Complete step by step answer:
Molarity and osmolarity are similar in a few aspects. Molarity is defined as the number of moles of solute present per liter of the solution. Osmolarity is the number of moles of osmoles present per liter of the solution.
The molarity is expressed as
$M = \dfrac{n}{V}$ where $n$ is the number of moles of solute and $V$ is the volume of the solution in liters. The unit of molarity is $mol{L^{ - 1}}$. The osmolarity is expressed as $osmol{L^{ - 1}}$.
Next is the relationship between osmolarity and molarity. An osmole of $1mole$ indicates that the substance is fully dissociated in the solvent. The osmolarity is referred to as the product of the molarity and the number of particles produced by the solute when dissolved in solution.
For example let us consider a solution of $NaCl$ as $1M$. The solution of $NaCl$ produces two particles in solution. Thus the molarity remains as $1M$ and the osmolarity is determined as
$= molarity \times number{\text{ }}of{\text{ }}particles$
$= 1M \times 2 = 2osmol{L^{ - 1}}$.
Thus counting the total number of particles released in the solution gives the osmolarity of the solution. The seawater is composed of various ions in the form of cations and anions. The cations are namely sodium, potassium, magnesium and calcium. The anions are namely chloride and sulphate.
Of these the most common ions are sodium and chloride. The ions are present in varying concentrations around the world.

Ion	Osmol/L
$N{a^ + }$	$545.8$
$C{l^ - }$	$468.0$
$S{O_4}^{2 - }$	$81.2$
$M{g^{2 + }}$	$53.1$
$C{a^{2 + }}$	$10$
${K^ + }$	$10$

Thus the osmolarity of sea water considering only sodium and chloride ions is about $1000osmol/L$. And the osmolarity is around $1200osmol/L$ considering all the ions.

Note: Both the osmolarity and the molarity indicate the concentration units. They only differ in the number of particles present in solution. The osmotic pressure of a solution is also determined using osmolarity.