SED & PREDOM / Ionic Strength

Concentration units

• molal = moles / (kg of water)

  Molality is therefore a concentration unit that is temperature- and pressure-independent.

• molar = moles / (1L of aqueous solution), is a concentration unit that is T- and p-dependent.

Given the density of the solution, ρ (g/cm³), and the molar mass ot the solute, M_s (g/mol), the molarity and molality may be calculated from each other:

	molar =	(1000 molal ρ) / (1000 + molal Ms)
	molal =	(1000 molar) / (1000 ρ − molar Ms)

At 25°C the density of dilute aqueous solutions is ≈1 g/cm³, and molality ≈ molarity; however the density varies with temperature, concentration, etc.

The densities of solutions of single salts may be calculated using the data in: Novotný P., Söhnel, O., Densities of binary aqueous solutions of 306 inorganic substances. J. Chem. Eng. Data, 33 (1988) 49–55. doi: 10.1021/je00051a018.

See also:

• activity
• activity coefficients
• ionic strength
• concentration units in diagrams