Van’t Hoff Factor and its Significance

 Van’t Hoff Factor: Definition

The Van’t Hoff factor is the ratio of final moles after dissociation or association to the initial moles before dissociation or association  of an electrolyte in a solution. The number of particles is independent of the concentration of the solution, and it is the property of the solute. The Van’t Hoff factor for a solution may be lower than the calculated value of the actual solution at a higher concentration, where the solute ions are associated with one another.

The Van’t Hoff factor is always a positive integer value; it can never be negative.

The Van’t Hoff factor is 1 when the solute remains completely undissociated in solution; it is greater than 1 for salts and acid and less than one for the solute that associates when dissolved to form a solution. Van’t Hoff factor applies mainly to colligative properties and is noticed in the formulas for the osmotic pressure, the vapour pressure of freezing point depression, and boiling point elevation.

Colligative Properties

It is the property of a solution that depends on the ratio of the number of solutes present to the number of solvent particles present in the solution; it does not depend upon the chemical nature of the solution. 

There are four main colligative properties:

1. Vapour pressure
2. Elevation of boiling point
3. Depression of freezing point
4. Osmotic pressure.

The particles that show colligative properties have an abnormal molar mass,  this abnormal molar mass is because of the dissociation of ions into the solution or association of the ions to form a larger molecule. This concept of abnormal molar mass was noticed as the experimental values obtained were different from the theoretical values.

The Degree of Association and Dissociation

The degree of dissociation is defined as the fraction of total molecules that dissociate into cation and anion.

The degree of association is defined as the fraction of total molecules that associate into forming larger molecules.

Formulae

To calculate the value of the Van’t Hoff factor there are different formulae used:

The formulas are based on molar mass and the colligative properties:

1.  i= Observed colligative property / Normal or theoretical colligative property.

1. i= Normal molar mass / Observed molar mass.

1.  i= Actual number of particles / Observed number of particles.

In the first formula, we are calculating the Van’t Hoff factor by using the colligative property.

In the second formula, we know that particles with colligative properties have an abnormal molar mass. By taking the values of the calculated molar mass and the experimentally observed molar mass, the Van’t Hoff’s factor can be calculated.

In the third formula, We are using the principle of colligative property as we know that colligative property is directly proportional to the number of particles present in the solution.

One of the most common formulas used to calculate the value of the Van’t Hoff factor is 

i = moles of particles in solution/moles solute dissolved.

Van’t Hoff Factor for the Association Solute

When the ions of solute associate in the solution they give values lesser than 1.

By taking an example of dimerization of acetic acid in presence of benzene, in this reaction, the acetic acid is dimerized into two molecules where the association of the ions takes place there for the Van’t Hoff factor is less than 1.

Van’t Hoff Factor for the Dissociation Solute

When the solute particles associate together in the solution they give values of more than.

By taking a sample of NaCl dissolving in water, NaCl is broken down into Na+ ions and Cl- ions.

Non-electrolyte

For non-electrolytes, the Van’t Hoff factor  is 1. Glucose sucrose fats Sugars all these are non-electrolytes and all of these have the Van’t Hoff factor as 1. In other words, they are completely dissolved in the solution.

Strong electrolyte

The strong electrolyte has a Van’t Hoff factor  greater than 1 and is equal to the number of ions formed in an aqueous solution when solutes dissociate, examples are strong acid bases and salts.

Weak electrolytes

As a weak electrolyte doesn’t completely dissociate into the solution to find the Van’t Hoff factor, in this case, is a bit different and it is not directly proportional to the number of ions formed.

Significance of Van’t Hoff Factor

The significant character of the Van’t Hoff factor is it is used in denoting the colligative properties. The formulas of colligative properties are included with the Van’t Hoff factor. 

• Elevation of the boiling point

  ΔTb = iKbm

• Depression at freezing point

ΔTf = iKfm

• Osmotic pressure

            π = iCRT

Where –  ΔTb =  elevation in boiling point

i- van’t hoff factor 

Kb – boiling point elevation constant

ΔTf = depression in freezing point

Kf – freezing point depression constant

m – molality of solution

 π = osmotic pressure 

C- concentration of solution

R- universal gas constant

T- temperature in kelvin

Conclusion

The Van’t Hoff factor is the ratio between the number of particles of solute and the number of particles of solvent. This value of toned independent of the concentration only depends on the number of particles of the solute. The application of colligative properties is seen in our daily life as it has some of the most important properties of how the solutes and Solvents interact.