• Engineering and Architecture
  • Management and Business Administration
  • Medicine and Allied Sciences
  • Law
  • Animation and Design
  • Media, Mass Communication and Journalism
  • Finance & Accounts
  • Computer Application and IT
  • Pharmacy
  • Hospitality and Tourism
  • Competition
  • School
  • Study Abroad
  • Arts, Commerce & Sciences
  • Learn
  • Online Courses and Certifications
Relation Between Molarity And Molality - Advantages, Fundamental Definitions, FAQs

Relation Between Molarity And Molality - Advantages, Fundamental Definitions, FAQs

Team Careers360Updated on 02 Jul 2025, 04:51 PM IST

Relation between molarity and molality

Molarity and molality are terminology used to indicate the concentration of a solution, or, more simply, the amount of solute contained in a solution. Before delving into the relationship between molality and molarity, we'll go over a few basic concepts and then develop a broad relationship between the two.

Advantages

The fundamental advantage of using molality as a measurement of concentration is that molality is determined only by the masses of solute and solvent, which are unaffected by temperature and pressure changes. Volumetric solutions (e.g., molar concentration or mass concentration) on the other hand, are likely to alter as temperature and pressure change. This is a considerable advantage in many applications since the mass, or amount, of a substance is frequently more essential than its area (e.g., in a limiting reagent problem).

Another benefit of molality is that the molality of one solute in a solution is unaffected by the presence or lack of other solutes.

Also read -

NEET Highest Scoring Chapters & Topics
This ebook serves as a valuable study guide for NEET exams, specifically designed to assist students in light of recent changes and the removal of certain topics from the NEET exam.
Download EBook

Origin

Molality is derived from molarity, which is the molar concentration of a solution. The intense property molality and its adjectival unit, the now-deprecated molal, appear to have been used for the first time by G. N. Lewis and M. Randall in their 1923 publication Thermodynamics and the Free Energies of Chemical Compounds. However, the two terms are sometimes confused, the molality and molarity of a dilute aqueous solution are almost identical, because one kilogramme of water (solvent) occupies the volume of one litre at room temperature and a tiny amount of solute has minimal effect on the volume.

Also Read:

Fundamental Definitions

A solution is made from a mixture, usually in liquid form, that consists of two components, namely a solute and a solvent. A solute is a little amount of a mixture's component, whereas the solvent is a big amount of the mixture's component.

A mole is a standard unit for expressing the amount of atoms or molecules of a substance present, such as water. One mole is represented by 12 grams of Carbon 12.

Molarity, also called molar concentration, is the number of moles of a solute in one litre of solution. M is used to represent it (capital m).

Molarity and molality formula is shown below.

Molarity = (number of moles of solute)/ (volume of solution (in litres)

Molality, on the other hand, is defined as the number of moles of solute contained in 1 kg (kilogramme) of solvent [The solution is made up of the solute and the solvent]. It is represented by the letter m. (small m).

Molality= (number of moles of solute)/ (weight of solvent (in kilogrammes))

Related Topics Link,

Molarity and molality relation

As we can see, both molarity and molality are important characteristics for determining the concentration of a solution; therefore, a relationship between them must be established in order to calculate one parameter with the help of the other. Using the following derivation, we will attempt to establish the link between molarity and molarity:

Assume the provided solute has a mass of W x gm (W/1000 kg).

The solute's molar mass is assumed to be M' x gm.

The solvent's weight is assumed to be W' x gm (W'/1000 kg).

The volume of the solution should therefore be V ml (V/1000 l).

The solution has a molality of m.

The solution's molarity is M.

As we all know, the number of moles is symbolised by the letter X, so:

X =? (mass of the so lute taken)/ (molar mass of the solute) =W/M (1)

As a result, molarity can be stated as follows:

M = (W/M’) (V/1000) = (W 1000)/ (M’ x V) ( 2 )

And molality can be written as:

(W/M')(W'/1000) = (W 1000)/(M'W') ( 3 )

Density = mass (of solute and solvent) divided by volume = (W+W')/V (4)

Equation (2) states:

V is = (W×1000)/(M'×M), and

We can deduct from equation (3):

W' = (W ×1000)/ (M'× m)

Hence,

W + W' = W + (W× 1000)/(M× m)

We get the following result when we split density by molarity:

(Density)/Molarity= (mass of solution in kilograms) (volume of solution in litres) )/(number of moles of solute)(volume of solution in liters)

= (mass of solution in kg)/ (number of moles of solute) (volume of solution in litres)) (number of moles of solute) (5)

Density/Molarity= (mass of solution in kg)/ (number of moles of solute) = (mass of solute in kg+mass of solvent in kg)/ (number of moles of solute)(6)

Equation (6) can also be written as follows: from equation 1

Density/Molarity=(mass of solution in kg)/(number of moles of solute)=(mass of solvent in kg)/(number of moles of solute)+ (W/1000 kg)/(W/M')

Density/Molarity=(mass of solution in kg)/(number of moles of solute)=(mass of solvent in kg)/(number of moles of solute)+ M'/1000 (7)

And because we know the molality formula, equation (7) may be written as,

Density/Molarity=(mass of solution in kg)/(number of moles of solute)=1/m+ M'/1000 (8)

As a result, density/molarity (d/M) is denoted as:

(Density) / (Molarity (M) = (mass of solution in kg)/(number of moles of solute)=1/m+ M'/1000

Molarity, molality, and their link is a very important topic that is frequently questioned in both subjective and objective questions, as well as numerical style.

Also check-

NCERT Chemistry Notes:

Commonly Asked Questions

Q: What is the fundamental difference between molarity and molality?
A:
Molarity is the number of moles of solute per liter of solution, while molality is the number of moles of solute per kilogram of solvent. The key difference lies in the reference: molarity uses the total solution volume, while molality uses only the solvent mass.
Q: Why does molality remain constant with temperature changes, but molarity doesn't?
A:
Molality remains constant because it's based on mass, which doesn't change with temperature. Molarity, however, is based on volume, which can expand or contract with temperature changes, affecting the concentration.
Q: In what situations is molality preferred over molarity?
A:
Molality is preferred in situations involving temperature changes, such as in colligative properties calculations, because it remains constant regardless of temperature. It's also useful when working with non-aqueous solvents or when precise masses are easier to measure than volumes.
Q: How does the density of a solution affect the relationship between molarity and molality?
A:
The density of a solution connects molarity and molality. For dilute aqueous solutions where the density is close to 1 g/mL, molarity and molality values are nearly equal. As the solution becomes more concentrated or if the solvent isn't water, the difference between molarity and molality increases.
Q: How does the molar mass of the solute affect the relationship between molarity and molality?
A:
For a given concentration, solutions with solutes of higher molar mass will have a larger difference between their molarity and molality values. This is because higher molar mass solutes contribute more to the solution's volume, affecting molarity more than molality.

Frequently Asked Questions (FAQs)

Q: How does the choice between molarity and molality affect the interpretation of spectroscopic titration data?
A:
In spectroscopic titrations, molarity is typically used because spectroscopic measurements are volume-based. However, when temperature effects are significant or when comparing data across different solvents, considering molality can provide insights into how the actual number of particles affects the spectroscopic properties, leading to more accurate interpretations.
Q: Why is understanding both molarity and molality important in the context of solution mixing and dilution processes?
A:
Understanding both molarity and molality is crucial in mixing and dilution processes because these operations can involve changes in both volume and mass. While molarity is often more convenient for volumetric operations, molality can provide a more consistent basis for calculations when temperature changes or non-additive volumes are involved.
Q: How does the concept of partial molar quantities relate to the difference between molarity and molality in solution thermodynamics?
A:
Partial molar quantities, which describe how thermodynamic properties change with the addition of a component at constant temperature and pressure, are more directly related to molality than molarity. This is because molality is based on a fixed amount of solvent, aligning better with the concept of adding small amounts of solute to a system.
Q: How does the choice between molarity and molality affect the calculation of reaction quotients and equilibrium constants in non-ideal solutions?
A:
In non-ideal solutions, using molalities (or activities based on molalities) for calculating reaction quotients and equilibrium constants often provides more accurate results than using molarities. This is because molality better represents the effective concentration of species in solution, especially when there are significant deviations from ideal behavior.
Q: How does the concept of limiting ionic conductivity relate to the choice between molarity and molality in electrolyte solutions?
A:
Limiting ionic conductivity, which describes the behavior of ions at infinite dilution, is typically expressed in terms of molarity. However, for more concentrated solutions or when comparing across different temperatures, using molality can provide a more consistent basis for understanding how ionic behavior changes with concentration.
Q: What is the importance of understanding both molarity and molality in the context of solution calorimetry?
A:
In solution calorimetry, understanding both molarity and molality is crucial. Molarity is often used for preparing solutions and calculating heat capacities per unit volume, while molality is useful for calculations involving energy changes per unit mass of solvent, especially when temperature changes are involved.
Q: How does the presence of electrolytes affect the relationship between molarity and molality?
A:
Electrolytes can significantly affect the relationship between molarity and molality, especially in concentrated solutions. The dissociation of electrolytes increases the number of particles in solution, affecting both the volume (relevant to molarity) and the solvent-solute interaction (relevant to molality) differently.
Q: Why is molality sometimes preferred in pharmaceutical formulations?
A:
Molality is often preferred in pharmaceutical formulations because it provides a more consistent measure of concentration regardless of temperature changes during manufacturing, storage, or administration. This is particularly important for maintaining the correct dosage and stability of medications.
Q: How does the choice between molarity and molality affect the calculation of equilibrium constants?
A:
Equilibrium constants are typically expressed in terms of activities, which are closer to molalities than molarities. In dilute solutions, molarities can be used as approximations. However, for more concentrated solutions or when high accuracy is required, using molalities (or activities based on molalities) can provide more accurate equilibrium constants.
Q: How does the concept of ideal dilute solutions relate to the interchangeability of molarity and molality?
A:
In ideal dilute solutions, the volume contribution of the solute is negligible, and the solution density is close to that of the pure solvent. Under these conditions, molarity and molality become nearly interchangeable because the mass of solvent (used in molality) is approximately equal to the volume of solution (used in molarity) divided by the solvent's density.
Articles
Previous
Next