Question

How can molarity and osmolarity be calculated from mass per unit volume?

Answer 1

Molarity is the number of moles of solute per litre of solution; osmolarity is the number of osmoles per litre of solution.

Assume you have a solution that contains 40.0 g of NaCl in 1.00 L of solution.

MOLARITY

The molarity of a solution is the number of moles of solute per litre of solution.

`M = n/V`

You have to convert the mass of NaCl to moles of NaCl.

Moles of NaCl = 40.0 g NaCl × `(1 mol NaCl)/(58.44 g NaCl)` = 0.684 mol NaCl

`M = n/V = (0.684 mol)/(1.00 L)` = 0.684 mol/L

OSMOLARITY

An ionic compound such as NaCl separates into cations and anions on dissolving.

NaCl(s) →Na⁺(aq) + Cl⁻(aq)

Osmosis depends on the number of solute particles in a solution. Both the number of moles of Na+ and the number of moles of Cl- contribute to the osmotic pressure. Thus, 1 mol of NaCl gives 2 mol of osmotic particles.

Each mole of solute that contributes to the osmotic pressure of a solution is an osmole.

The osmolarity is the molarity of the compound times the number of particles produced by one mole of compound.

The osmolarity of the NaCl solution is

`(0.684 mol)/(1 L) × (2 Osmol)/(1 mol)` = 1.37 Osmol/L