# How can I calculate osmolarity of NaCl?

Dec 7, 2015

This is simply twice the molarity of $N a C l \left(a q\right)$.

#### Explanation:

Osmolarity is simply the concentration of all ions in solutions. It is thus the sum of sodium ion and chloride ion concentrations, inasmuch as as sodium chloride is a strong electrolyte, and dissolves to give stoichiometric $N {a}^{+} \left(a q\right)$ and $C {l}^{-} \left(a q\right)$ ions.

Jan 14, 2016

You multiply the molarity of the $\text{NaCl}$ by $2$.

#### Explanation:

An osmole (Osmol) is a mole of particles that contribute to the osmotic pressure of a solution.

For example, $\text{NaCl}$ dissociates completely in water to form ${\text{Na}}^{+}$ ions and ${\text{Cl}}^{-}$ ions.

"NaCl(s)" → "Na"^+("aq") + "Cl"^(-)("aq")

Each ion contributes to the osmotic pressure of the solution.

Thus, each mole of $\text{NaCl}$ becomes two osmoles in solution: one mole of ${\text{Na}}^{+}$ and one mole of ${\text{Cl}}^{-}$.

A solution of 1 mol/L $\text{NaCl}$ has an osmolarity of 2 Osmol/L.

EXAMPLE

Calculate the osmolarity of 0.140 mol/L $\text{NaCl}$.

Solution

$\text{[NaCl] = 0.140 mol/L}$

Each mole of $\text{NaCl}$ becomes two osmoles.

$\text{Osmolarity" = (0.140 color(red)(cancel(color(black)("mol"))))"/L" × "2 Osmol"/(1 color(red)(cancel(color(black)("mol")))) = "0.280 Osmol/L}$