Abnormal Molar Mass
A molar mass value obtained from colligative measurements that differs from the true value because the solute associates or dissociates.
Formula / equation
M_normal = i × M_observed
Unit
g/mol
In depth
Because colligative properties count particles rather than molecules, a solute that splits into ions appears to have a molar mass smaller than the true value, while one that dimerises appears larger. Applying the van't Hoff factor corrects the result to the true molar mass.
Examples in the real world
KCl in water appears to have a molar mass near 37 g/mol; acetic acid in benzene appears to be near 120 g/mol.