Abnormal Molar Mass and van't Hoff Factor
Association and dissociation corrections. (Chemistry › Solutions, NEET UG syllabus.)
What is Abnormal Molar Mass and van't Hoff Factor?
The molar mass of a solute calculated from observed colligative properties that deviates from its theoretically expected or normal molar mass due to association or dissociation of solute particles in the solution.
Key formula / rule: Relative Lowering of Vapour Pressure (Modified)
Key points
- Define abnormal molar mass and explain its origin.
- Define the van't Hoff factor (i) and explain its significance.
- Calculate 'i' from observed and theoretical colligative properties or molar masses.
- Apply 'i' to modify colligative property equations for solutions with association or dissociation.
Common exam trap
Forgetting to apply 'i' for electrolytes or associating solutes.
Definitions
- Term
Abnormal Molar Mass
- Meaning
The molar mass of a solute calculated from observed colligative properties that deviates from its theoretically expected or normal molar mass due to association or dissociation of solute particles in the solution.
- Term
van't Hoff Factor (i)
- Meaning
A dimensionless factor that accounts for the effect of solute association or dissociation on colligative properties, defined as the ratio of the observed colligative property to the theoretical colligative property, or the ratio of normal molar mass to abnormal molar mass.
Learning objectives
Define abnormal molar mass and explain its origin.
Define the van't Hoff factor (i) and explain its significance.
Calculate 'i' from observed and theoretical colligative properties or molar masses.
Apply 'i' to modify colligative property equations for solutions with association or dissociation.
Relate 'i' to the ° of dissociation (α) for electrolytes.
Relate 'i' to the ° of association (α) for associating solutes.
Predict the value of 'i' for different types of solutes (strong electrolytes, weak electrolytes, associating solutes, non-electrolytes).
Formulae
- Name
Relative Lowering of Vapour Pressure (Modified)
- Note
P° is vapor pressure of pure solvent, P is vapor pressure of solution, Xsolute is mole fraction of solute, i is van't Hoff factor.
- Expression
(P° - P) / P° = i * Xsolute
- Name
Elevation in Boiling Point (Modified)
- Note
ΔTb is elevation in boiling point, Kb is molal elevation constant, m is molality of solution, i is van't Hoff factor.
- Expression
ΔTb = i * Kb * m
- Name
Depression in Freezing Point (Modified)
- Note
ΔTf is depression in freezing point, Kf is molal depression constant, m is molality of solution, i is van't Hoff factor.
- Expression
ΔTf = i * Kf * m
- Name
Osmotic Pressure (Modified)
- Note
π is osmotic pressure, C is molar concentration, R is gas constant, T is temperature in Kelvin, i is van't Hoff factor.
- Expression
π = i * C * R * T
- Name
van't Hoff Factor (General Definition)
- Note
Relates experimental observation to theoretical prediction.
- Expression
i = (Observed Colligative Property) / (Calculated Colligative Property)
- Name
van't Hoff Factor (Molar Mass Relation)
- Note
Abnormal molar mass is calculated from observed colligative property.
- Expression
i = (Normal Molar Mass) / (Abnormal Molar Mass)
- Name
van't Hoff Factor (for Dissociation)
- Note
α is ° of dissociation, n is number of ions produced per molecule.
- Expression
i = 1 + α(n-1)
- Name
van't Hoff Factor (for Association)
- Note
α is ° of association, n is number of molecules associating.
- Expression
i = 1 + α(1/n - 1)
Prerequisites
Understanding of colligative properties (RLVP, ΔTb, ΔTf, π).
Knowledge of molar mass and mole concept.
Basic understanding of electrolytes and non-electrolytes.
Concept of dissociation and association.
Common mistakes
Forgetting to apply 'i' for electrolytes or associating solutes.
Incorrectly calculating 'i' for partial dissociation/association.
Confusing observed and theoretical values in 'i' calculations.
Assuming 'i' is always an integer for electrolytes (it's often slightly less due to interionic attractions).
Not understanding that 'i' is used to *correct* the number of particles, not the nature.
Keywords
van't Hoff factor
abnormal molar mass
colligative properties
dissociation
association
° of dissociation
° of association
electrolyte
non-electrolyte
osmotic pressure
boiling point elevation
freezing point depression
relative lowering of vapor pressure
Practice preview
What is the van't Hoff factor (i) for a solute that undergoes complete dissociation in a solution?…
easy
How does the van't Hoff factor (i) modify the expression for colligative properties like elevation in boiling point (delta_Tb)?…
easy
A solution of a non-volatile solute has an observed molar mass of 60 g/mol. If its normal molar mass is 30 g/mol, what is the van't Hoff factor (i) for this solute?…
medium