Molality

Molality is the unit of several moles of a solution corresponding to 1kg or 1000 g solvent. This molality is based on a specific volume of solution. Molality is a commonly used unit in chemistry, mostly known as mol or kg. A concentration of 1mol/ kg is also known as 1 molal. However, the molar mass is expressed in gmol or kmol instead of kg/mol or g/mol. This molality study material is designed to provide a proper guideline to the students preparing for IIT JEE. These study material notes on molality will cover the basic concept of molality and its usage consideration on the usage of chemistry.

Origin 

The term molality is formed from molarity, which means the molar concentration of a solution. This was earlier used as an intensive property of Molality and its adjectival unit. It appears in the publication of thermodynamics and the free energies of chemical substances by GS Lewis and M. Randall in 1923. Molality and molarity are the two terms that are a little confusing as their usage in solutions is almost the same.  

Unit 

There are two most used units of molality. Mol/kg is the unit of molality. It means moles per kilo solution or 1 mol/kg is called 1 molal or 1m. The SI unit of molality is mole per kilogram of solvent. For example, a solution of 3mol /kg can be determined as 3 molal or 3m.

Advantages and disadvantages 

Advantages

• Molality only depends on the mass of solution and solvent, and it is unaffected by various pressure and temperature. However, the volumetrically created solution changes the concentration as the pressure and temperature change. This is an essential advantage of using molality as a unit.
• The molality of one solution does not depend on another presence or absence of solute. 
• Molality is one of the essential components of a solution. It is made to express the solution’s concentration that depends on the solvent’s mass. 

This unit is not applicable where no pure substance is present in the solution.

Disadvantages 

One of the significant disadvantages of Molality is observed in mixtures that do not contain pure substances. Here it is hard to distinguish between the solute and solvent. One example is a mixture of alcohol and water or alcohol, water and alloys. In such a solution, the solvent and solute can be interchanged.

Difference between molality and molarity 

Molality is based on a kilogram of solvent, whereas molarity is based on litres of solution. In molality, the concentration of the solution is expressed as a ratio of solute per 1000 g of solvent. Temperature changes do not affect molality. The unit of molality is mol/kg and denoted by ‘m’.

On the other hand, molarity is the concentration of solute in a solution. Therefore, temperature changes directly affect the molarity of any substance. The unit of molarity is mol/litre and denoted by ‘M”. 

Molality and molarity are similar only in the case of dilute solutions. For example, adding a low quantity of solute in water has a negligible effect on the molar or molal values. It is because, at room temperature, 1 Kg of water occupies 1 litre of volume.

Relation between molality and molarity using density

The relation between Molality (m) and molarity (M) in terms of Density is given as:

m = (1000M) / (1000p-MM1)

Here, the parameters are:

p = Solution density in mg/mL

M1 = Solute’s molecular weight

Molality= moles of solute/ kilograms of solvent

It is easy to calculate molality if we know the mass of solute and solvent in a solution. However, it is an intensive property and can be measured independently. 

Molality depends on the mass of the solvent, but molarity relies on the volume of the solution. The changes in pressure and temperature can change the volume. Therefore, molarity can also be changed depending on pressure, unlike molality. However, molality does not change with the temperature changes. Therefore, molality is more convenient to use while working with the temperature ranges.