Effective van ’t Hoff factors (25 °C)

Effective van ’t Hoff factors i = νφ computed from Pitzer osmotic coefficients. In dilute solution i falls short of the ion count ν, most of all for doubly charged ions (MgSO₄); in concentrated solution hydration pushes it back up.


Sodium chloride	$\ce{NaCl}$	2	1.936	1.864	1.842	1.872	1.969
Potassium chloride	$\ce{KCl}$	2	1.935	1.853	1.800	1.797	1.826
Calcium chloride	$\ce{CaCl2}$	3	2.721	2.566	2.745	3.142	4.155
Magnesium sulfate	$\ce{MgSO4}$	2	1.482	1.191	1.053	1.056	1.323

i = νφ is exactly the factor that freezing-point, boiling-point and osmotic-pressure experiments “measure”. Upper limits of the Pitzer parameter sets: NaCl 6, KCl 4.8, CaCl₂ 2.5, MgSO₄ 3 mol/kg.

Sources

K. S. Pitzer & G. Mayorga, J. Phys. Chem. 77, 2300 (1973) (NaCl, KCl, CaCl₂); J. Solution Chem. 3, 539 (1974) (MgSO₄).
R. A. Robinson & R. H. Stokes, Electrolyte Solutions, 2nd ed. (1959): measured osmotic coefficients used to fit those parameters.

Used in these lessons

Colligative properties: freezing, boiling and osmosis