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Molality (m) and molarity (M) are two terms that describe the concentration of a chemical solution. The amount of moles of solute per kilogramm of solvent is referred to as molality. The number of moles of solute per litre of solution is referred to as molarity. When the solvent is water and the concentration of the solute is low (i.e., dilute solution), molality and molarity are about equal. The approximation, however, fails if a solution grows more concentrated, contains a solvent other than water, or endures temperature changes that might affect the density of the solvent. Because the mass of solute and solvent in a solution does not vary in these cases, molality is the preferable technique of expressing concentration.
The usage of molarity is quite convenient for many reasons. When we wish to determine the concentration of a solute in a scenario where the temperature varies, molarity is ineffective. Temperature causes a little change in the volume of the solution, which affects the accuracy of data observations and computations. Another parameter is required, one that is unaffected by the temperature of the substance under consideration and this is when molality comes into play.
Molality is an important property of solutions. It is used to indicate the concentration of a solute in a solution and is mostly determined by the solvent’s mass. Molality is also known as the molal concentration at times. It is commonly represented by the character “m.”
Molality is the number of moles of solute in a solution equal to 1 kilogram or 1000 g of solvent. This is in contrast to the concept of molarity, which is based on the volume of a solution.
Molality was originally defined as a relationship to molarity, which is defined as the molar concentration of a solution. The usage of this property was originally documented in a 1923 paper by G. N. Lewis and M. Randall. The book “Thermodynamics and the Free Energies of Chemical Substances” discussed the subject.
The sole difference between molality and molarity is in the denominator. Unlike molarity, which is measured in litres of solution, molality is measured in kilogram of solvent. When examining the characteristics of solutions associated to vapour pressure and temperature changes, concentrations given in molality are utilized. Molality is employed since its value does not vary as a result of temperature variations. The volume of a solution, on the other hand, varies somewhat with temperature.
Definition and Formula for Molality
The amount of moles of solute per kilogram of solvent is known as molality (m). The molality formula is as follows:
Molality (m) = moles of solute divided by kilogram of solvent
m = mol/kg
Students must keep in mind that molality is used to calculate moles in proportion to the mass of the solvent rather than the mass of the solution.
There are also situations when the solutions contain more than one solvent. In such circumstances, the molality of the combined solvent is taken into account or defined. The units can also be represented as moles of solute per kilogram of mixed solvent.
SI unit of Molality
Molality is measured in moles per kilogram (mol/kg) in the SI system. For example, a solution with a molality of 15 mol/kg is written as 15 molal or 15 m.
The Benefits and Drawbacks of Using Molality
One of the benefits of using molality as a measure of concentration is that, as previously stated, molality is simply reliant on the mass of solute and solvent. This means that changes in temperature or even pressure have no effect on these parameters, as in the case of volumetrically prepared solutions. Second, it is particularly beneficial since the molality of one solute in a solution is independent of the presence of other solutes.
Not terms of drawbacks, it is inapplicable when there is no pure ingredient in a combination. For example, water and alcohol mixes or alloys. In this case, any of the substances might be regarded as the solvent.
The molality of a solution does not change with temperature. Molality is defined as the ratio of moles to mass. Because mass remains constant at all temperatures, molality does not change with temperature.
The colligative characteristics of a solution are determined by the number of particles present. Molality is defined as the kilos of solvent that stay constant with regard to temperature. As a result, molality is employed in colligative characteristics.
Molality is employed whenever there is a possibility that the solute will interact with the solute, as well as in the following situations:
- To ascertain a boiling point
- To ascertain a melting point
- When it comes to colligative qualities (boiling point elevation, freezing point depression)
In general, the difference between molarity and molality for aqueous solutions around room temperature is quite tiny, and it makes little difference whether you use a molar or a molal concentration.
FAQ’s
Q. What is the distinction between molality and molarity?
ANS: Molality and molarity do not vary greatly in a dilute aqueous solution, but they do in concentrated solutions. The following are the primary points of distinction between them:
- Molarity is determined by the mass of solvent in the solution, whereas molarity is determined by the volume of the solution.
- Molality is denoted by m, whereas molarity is denoted by M.
- Molality is measured in moles per kilogram, whereas molarity is measured in moles per litre.
- The change in temperature of the solution has no effect on molality, however the change in temperature of the solution has an effect on molarity.
How Do You Work Out Molality?
Molality is mostly used to express the concentrations of solutions in relation to changes in vapour pressure and temperature. Molality is also used to determine the boiling or melting point, as well as when working with colligative qualities. In terms of math, molality may be simply estimated by knowing the masses of solute and solvent in a solution. Furthermore, the concentration or molality of a homogeneous solution is always constant.
Is molality impacted by temperature changes?
Because molality is defined in terms of the mass of the solvent rather than its volume, the temperature has no effect on the molality of a solution.
