Question

How can I calculate osmolarity of NaCl?

Answer 1

This is simply twice the molarity of `NaCl(aq)`.

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 `Na^(+)(aq)` and `Cl^(-)(aq)` ions.

Answer 2

You multiply the molarity of the `"NaCl"` by `2`.

Explanation:

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

For example, `"NaCl"` dissociates completely in water to form `"Na"^+` ions and `"Cl"^-` ions.

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

Each ion contributes to the osmotic pressure of the solution.

Thus, each mole of `"NaCl"` becomes two osmoles in solution: one mole of `"Na"^+` and one mole of `"Cl"^-`.

A solution of 1 mol/L `"NaCl"` has an osmolarity of 2 Osmol/L.

EXAMPLE

Calculate the osmolarity of 0.140 mol/L `"NaCl"`.

Solution

`"[NaCl] = 0.140 mol/L"`

Each mole of `"NaCl"` becomes two osmoles.

∴ `"Osmolarity" = (0.140 color(red)(cancel(color(black)("mol"))))"/L" × "2 Osmol"/(1 color(red)(cancel(color(black)("mol")))) = "0.280 Osmol/L"`