Question

Define the following terms:
(i) Molality (m)
(ii) Abnormal Molar mass

Answer 1

Hint: .Molality and Abnormal molar mass are the terms taken into discussion when we talk about concentration or colligative properties. The term Abnormal molar mass is related to the Van't Hoff factor.


Complete answer:
Let us discuss Molality first;
(i) Molality: Molality is also known as molal concentration, is a measure of the concentration of a solute in the solution in terms of amount of substance in specified mass of the solvent. It is denoted by (m). Molality is an intensive property. It is a property of a solution. Molality can also be defined as the number of moles of solute present per kilogram of solvent. Mathematically, it can be represented as m=$\dfrac{\text{number of moles of substance}}{\text{mass of solvent(kg)}}$. The SI unit of molality is moles/kg. A solution having molality of 2 mol/kg can be written as ‘2 molal’ or ‘2m’. 1 molal can be read as ‘1 mole of solute present in 1 kg of solvent’.
Let us see Abnormal molar mass now;
(ii) Abnormal Molar mass: The name itself suggests when the molar masses need to be calculated and the value obtained has higher or lower than the expected value are known as abnormal molar masses. This abnormality in molar mass is being calculated using the Van’t Hoff factor. Molar mass of solute is inversely proportional to colligative properties. So, decrement in colligative properties leads to an increase in the molecular mass. There are two situations in which abnormal molar mass is observed
- Association of ions: Ethanoic acid associates when dissolved in benzene dimerizes because the number of particles and hydrogen bonding is reduced. This experiment increases molar mass more than the expected value.
- Dissociation of ions: Weak electrolytes are dissolved in the water and ions are formed. Hydrochloric acid will dissociate into chlorine ion and hydrogen ion.
The relation of Van’t Hoff factor with Abnormal molar mass is:
$\text{Van }\!\!'\!\!\text{ t Hoff factor}=\dfrac{\text{abnormal mass}}{\text{molar mass}}$

Additional Information: Molality is equal to molarity when the solution is very dilute means the volume of solution can be considered equal to its mass of solution. The solution is dilute so, the amount of solvent is very large in comparison to the amount of solute. So, when the solution is given dilute then, consider molality equal to Molarity (m=M).

Note: The term molality and molarity deals with the concentration of solute in the solution. So, sometimes there is a point of confusion between the two. But, there is difference between molality and molarity:

MOLARITY	MOLALITY
It is moles of solute dissolved in 1 litre of solution.	It is moles of solute dissolved in 1 kg of solvent.
Symbol is ‘M’.	Symbol is ‘m’.
Unit is mole/litre.	Unit is mole/kg.
It is affected by changes in temperature and pressure.	It is not affected by changes in temperature and pressure.
The values obtained may be inaccurate.	The values obtained are very accurate and precise.