Understanding the Van’t Hoff factor

The Van’t Hoff factor explains how solutes affect the colligative characteristics of solutions, that are: 

1. The depression in freezing point, elevation in boiling point, vapour pressure depression, and osmotic pressure 
2. The Van’t Hoff factor is denoted by i. It is defined as the ratio of the direct concentration of the particle that is created after the dissolution to the concentration calculated from the mass of the material.

i =n+(1-)

Where i = Van’t Hoff factor

= degree of dissociation

n = number of ions formed from one formula unit of the substance

3. It is the number of ions a solute dissolves into per formula unit. Due to this fact, a change in a colligative property is equal to the solute particle  number in solution (a solute that dissociates into numerous ions will result in a bigger change). It can be accounted for using the Van’t Hoff factor.
4. The van ‘t Hoff factor is the ratio between the actual concentration of particles produced when the substance is dissolved and the concentration of a substance as calculated from its mass.

Van’t Hoff factor formula 

Van’t Hoff factor (i) was introduced in 1880, by Van’t Hoff, for the expansion of dissociation and association. 

The formula of van’t Hoff factor is 

i = normal molar massabnormal molar mass

i = observed colligative propertycalculated colligative property

i = the amount (or total number of moles) of particles after association or dissociation number of moles of particles before association or dissociation

These are the formulas for the van’t Hoff factor. By these formulas, colligative properties can be obtained or calculated. The Van’t Hoff factor (i) is multiplied by the equations of colligative properties.

Van’t Hoff factor for aluminium sulphate under complete dissociation

Aluminium sulphate is a white powder or crystal. It is a salt and also known as astringent. It is miscible in water and used in purifying it. Its chemical formula is Al2(SO4)3.

When a molecule of a solute dissociates in a solvent, the Van’t Hoff factor equals the number of ions created. On complete dissociation of Al2(SO4)3There are 2 ions of aluminium and 3 ions of sulphate. Thus, for aluminium sulphate, the Van’t Hoff factor is 5.

Al2(SO4)3 —> 2Al3+ + 3SO42–

Abnormal molar mass and Van’t Hoff factor 

When calculated from the colligative properties of a solution, abnormal molar mass refers to theoretical molecular mass values that differ from empirically determined values.

According to Van’t Hoff, when substances are dissolved in a solvent, they completely dissolve and break into ions. Because colligative properties are exclusively defined by the number of solute particles, breaking down solute molecules into ions increases the number of particles and hence has an impact on colligative capacities.

Calculation of abnormal molar masses is done by colligative properties. The molar masses can be higher or lower than expected values, and that is why they are abnormal. They can be calculated by using the Van’t Hoff factor.

When a chemical compound is dissolved in water, all the molecules of that substance dissociate in water.

E.g.:

 Suppose NaCl is dissolved in water, one ion of sodium and one ion of chloride will be dissociated – total 2 moles of ions. But when calculating, 1 mol of NaCl is taken as present in the solution.

The reasons for abnormality in molecular mass are the following:

The value of the colligative property increases due to the increase in the particles’ number. The particles’ number increases by the dissociation of substances into ions.

The overall particles’ number in a solution drops when solute particles connect with one another, resulting in a decrease in colligative characteristics.

Conclusion:

The Van’t Hoff factor got its name from the Dutch scientist Jacobus Henricus Van’t Hoff, who won the first Nobel Prize in chemistry. The Van’t Hoff factor is used to measure the effects on colligative properties. The extent of a material decomposition or its correlation with a solution is described by the Van’t Hoff factor. The value of “i” is normally 1 when a non-electrolytic substance is dissolved in water because it doesn’t decompose. For example, ammonia. When an ionic molecule forms a solution in water, the value of “i” will be equal to the number of ions present in one formula unit of the ionic molecule. This factor also helps in calculating the colligative properties of solutions and abnormal molar mass. The calculated molar mass is different than expected due to the dissociation of each particle in the solution. The Van’t Hoff factor also helps in calculating the exact molar mass.