Relation Between Molarity And Molality

Edited By Team Careers360 | Updated on Jul 02, 2025 04:51 PM IST

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:

Frequently Asked Questions (FAQs)

1. Determine the relationship between molarity and molarity.

The student is expected to provide all of the points indicated above under the subheading: Relationship between molarity and molality.

2. What exactly is molarity?

Molarity, also known as molar concentration, is the number of moles of a solute in one litre of solution. M is used to represent it.

3. What is the molality of a sodium sulphate solution containing 138 gm sodium sulphate ions per kg water (solvent)?

A complete answer to the question

The given weight of sodium ions is 138 grammes.

Sodium has a molecular mass of 23 grammes.

Number of sodium ion moles = 138/23 = 6 moles

1 kg is the weight of the given solvent.

Hence,

Molality is defined as the number of moles of sodium ion divided by the weight of the solvent in kilogrammes.

6/1 =

6 m

4. Which is higher in concentration: 1M or 1m?

1M stands for 1 molar solution, which includes 1 mole of solute in a 1-litre solution that contains both solute and solvent. 1m, on the other hand, denotes 1 molal solution, which includes 1 mole of solute in 1 kilogramme of solvent, which is approximately 1 litre. We can easily see that 1m solution contains more solvent than 1M solution, and both contain 1-mole solute. As a result, 1M has a higher concentration than 1m.

5. How does temperature affect molarity or molar relationship with temperature?

We know that molarity is the number of moles of solute per litre of solution, and that as the temperature rises, so does the volume. As a result, the molarity falls, and molarity becomes temperature dependent.

6. Which of the following is pressure dependent: molality or molarity?

Molality is determined only by the mass of the solvent, whereas molarity is determined by the volume of solution, which may be altered by changing the pressure and temperature. In addition, unlike molality, molarity is pressure dependent.

7. When working with a temperature range, which is preferable: molality or molarity?

We know that molality does not alter as a function of temperature. As a result, when working with a wide variety of temperatures, molality will be more convenient to utilise.

8. What is the metric unit for mole fraction?

This is unitless since it is the ratio of moles to moles, which are also unitless separately.

9. What exactly is normalcy?

Normality is another method for calculating the concentration of a solution. It is the gramme equivalent weight of the solute per litre of solution. Its SI unit is eq / lt, and the letter 'N' stands for normality.

10. What is the fundamental difference between molarity and molality?

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.

11. Why does molality remain constant with temperature changes, but molarity doesn't?

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.

12. In what situations is molality preferred over molarity?

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.

13. How does the density of a solution affect the relationship between molarity and molality?

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.

14. How does the molar mass of the solute affect the relationship between molarity and molality?

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.

15. What is the main advantage of using molarity in laboratory settings?

The main advantage of using molarity in labs is its practicality. It's easier to measure and dispense volumes of solutions than to weigh out specific masses, making molarity more convenient for preparing and using solutions in experimental procedures.

16. What is the significance of the number 1000 in the molality formula?

The number 1000 in the molality formula (m = moles of solute / kg of solvent) represents the conversion factor from grams to kilograms. It ensures that molality is expressed in moles per kilogram, maintaining consistency in units.

17. Why is molality particularly useful in vapor pressure calculations?

Molality is useful in vapor pressure calculations because it directly relates to the ratio of solute to solvent particles, which determines the vapor pressure lowering. Since molality is based on the mass of solvent, it provides a more direct link to the number of solvent molecules available for vaporization.

18. How does the concept of osmolality relate to molality, and why is it important in biological systems?

Osmolality is closely related to molality but accounts for all particles in solution, including those from dissociation. It's important in biological systems because it determines osmotic pressure, which is crucial for cellular processes. Osmolality is often used in medical contexts because it's more directly related to the body's physiological state than molarity.

19. What is the significance of using molal solutions in freezing point depression experiments?

Molal solutions are ideal for freezing point depression experiments because molality directly relates to the number of solute particles per kilogram of solvent, which determines the extent of freezing point depression. Since molality is independent of temperature, it provides consistent results even as the solution cools.

20. Why is molality particularly useful in studying non-volatile solutes?

Molality is particularly useful for non-volatile solutes because it focuses on the ratio of solute to solvent, ignoring any volume changes. This is especially relevant when studying properties like boiling point elevation, where the non-volatile solute's effect on the solvent's behavior is of primary interest.

21. How does the concept of activity coefficients bridge the gap between ideal behavior (represented by molality) and real solution behavior?

Activity coefficients adjust molal concentrations to account for non-ideal interactions in real solutions. They represent the ratio of the actual activity of a species to its molal concentration, allowing thermodynamic equations based on ideal behavior to be applied to real solutions by using these adjusted concentrations.

22. Why is molality particularly useful in studying the properties of mixed solvents?

Molality is useful for mixed solvents because it's based on the total mass of the solvent mixture, regardless of how the individual solvent components interact or how their volumes combine. This makes molality a more straightforward and consistent measure when dealing with complex solvent systems.

23. How does the concept of mole fraction relate to molality, and why is this relationship important?

Mole fraction and molality are closely related, as both are based on the number of moles of components rather than volume. The relationship between them is important because it allows for easy conversion between different concentration units and is particularly useful in thermodynamic calculations involving mixtures.

24. What is the significance of using molal concentrations in electrochemistry, particularly in electrode potential calculations?

In electrochemistry, molal concentrations are significant for electrode potential calculations because they provide a more fundamental representation of solution composition, especially when dealing with non-ideal or concentrated solutions. Molality is less affected by temperature and pressure changes, which can be important in electrochemical systems.

25. Why is molality particularly useful in studying the properties of volatile solutions?

Molality is useful for volatile solutions because it remains constant even as the solvent evaporates. This is particularly important in studies involving vapor pressure or distillation, where the concentration in the liquid phase needs to be accurately known despite ongoing changes in volume due to evaporation.

26. What is the significance of using molal concentrations in the study of membrane equilibria and osmotic systems?

Molal concentrations are significant in membrane equilibria and osmotic systems because these processes depend on the number of particles per unit mass of solvent, which molality directly represents. This makes molality particularly useful for calculating osmotic pressures and understanding the behavior of solutions across semipermeable membranes.

27. Why is molality used in freezing point depression and boiling point elevation calculations?

Molality is used in these calculations because colligative properties depend on the ratio of solute particles to solvent particles, which is directly related to molality. Additionally, molality remains constant with temperature changes, making it more reliable for these temperature-dependent properties.

28. How can you convert between molarity and molality?

To convert between molarity and molality, you need to know the density of the solution and the molar mass of the solute. The general formula is: Molality = Molarity * (solution density) / (1000 - (Molarity * solute molar mass)). This conversion accounts for the volume-mass relationship in the solution.

29. How does the concept of ideal solutions relate to molarity and molality?

In ideal solutions, the volume of the solution is assumed to be the sum of the volumes of its components. This assumption works well for dilute solutions where molarity and molality are nearly equal. However, in non-ideal solutions, especially at higher concentrations, this relationship becomes more complex.

30. Why is molality sometimes referred to as a "more fundamental" unit than molarity?

Molality is considered more fundamental because it depends only on the number of particles and is independent of temperature and pressure. This makes it a more consistent measure of concentration across different conditions, especially in thermodynamic calculations.

31. How does the choice between molarity and molality affect stoichiometric calculations?

In stoichiometric calculations, molarity is often more convenient because reactions are typically carried out with volumes of solutions. However, if temperature changes are involved, using molality can provide more accurate results as it remains constant with temperature.

32. What role does the solvent play in determining whether to use molarity or molality?

The choice between molarity and molality can depend on the solvent. For aqueous solutions at room temperature, both are often interchangeable. However, for non-aqueous solvents or solutions with significant thermal expansion, molality is generally preferred due to its independence from volume changes.

33. How does the concept of activity relate to molarity and molality in non-ideal solutions?

In non-ideal solutions, the actual behavior of solutes deviates from what molarity or molality alone would predict. Activity, which is an effective concentration, accounts for these deviations. It's often expressed in terms of molality but adjusted with an activity coefficient to reflect real solution behavior.

34. How does the concept of infinite dilution relate to the difference between molarity and molality?

At infinite dilution, where the solution is extremely dilute, the difference between molarity and molality becomes negligible. This is because the volume contribution of the solute becomes insignificant, and the density of the solution approaches that of the pure solvent.

35. What is the significance of using molal solutions in standardization procedures?

Molal solutions are useful in standardization procedures because their concentration remains constant regardless of temperature. This is particularly important when precise concentrations are needed across different environmental conditions or when temperature control is difficult.

36. How does the choice between molarity and molality affect the calculation of reaction rates?

Reaction rates are typically expressed in terms of molarity because reactions occur in a volume of solution. However, if significant temperature changes occur during the reaction, using molality can provide more consistent results, as it remains unaffected by volume changes due to temperature.

37. Why is molality preferred in thermodynamic calculations involving solutions?

Molality is preferred in thermodynamic calculations because it's based on the mass of the solvent, which doesn't change with temperature or pressure. This makes it more suitable for calculations involving energy changes, entropy, and other thermodynamic properties that are sensitive to temperature and pressure variations.

38. How does the concept of partial molar volume relate to the difference between molarity and molality?

Partial molar volume, which is the change in solution volume when adding a small amount of component at constant temperature and pressure, affects molarity but not molality. This concept explains why molarity can change even when the number of moles remains constant, while molality remains unaffected.

39. What is the importance of specifying whether a concentration is molar or molal in scientific communication?

Specifying whether a concentration is molar or molal is crucial in scientific communication to ensure accurate replication of experiments and correct interpretation of results. The two units can lead to significantly different values, especially in concentrated solutions or non-aqueous systems.

40. How does the presence of electrolytes affect the relationship between molarity and molality?

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.

41. Why is molality sometimes preferred in pharmaceutical formulations?

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.

42. How does the choice between molarity and molality affect the calculation of equilibrium constants?

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.

43. How does the concept of ideal dilute solutions relate to the interchangeability of molarity and molality?

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.

44. What is the significance of using molal concentrations in colligative property studies?

Molal concentrations are significant in colligative property studies because these properties depend on the ratio of solute particles to solvent molecules, which is directly related to molality. Using molality simplifies calculations and provides more consistent results across different temperatures and pressures.

45. How does the choice between molarity and molality affect the interpretation of spectroscopic data?

In spectroscopy, molarity is often used because spectroscopic measurements typically depend on the number of absorbing species per unit volume. However, when temperature effects are significant or when comparing data across different solvents, molality can provide a more consistent basis for interpretation, especially in quantitative analyses.

46. Why is molality preferred in studying the properties of non-aqueous solutions?

Molality is preferred for non-aqueous solutions because it's independent of the solvent's thermal expansion, which can be significant and varied for different non-aqueous solvents. This makes molality more reliable for comparing concentrations and properties across different solvent systems.

47. What is the importance of understanding both molarity and molality in the context of solution calorimetry?

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.

48. How does the solvent's molecular weight affect the relationship between molarity and molality?

The solvent's molecular weight affects the relationship between molarity and molality because it influences the solution's density and the mass-volume relationship. For solvents with higher molecular weights, the difference between molarity and molality for a given solution tends to be larger than for solvents with lower molecular weights.

49. How does the choice between molarity and molality affect the interpretation of colligative property measurements in polymer solutions?

In polymer solutions, molality can be more appropriate for interpreting colligative property measurements because it directly relates to the number of solute particles, which is crucial for these properties. Molarity can be misleading in polymer solutions due to the large volume occupied by polymer molecules and potential volume changes with concentration.

50. Why is understanding the relationship between molarity and molality important in the context of solution thermodynamics?

Understanding the relationship between molarity and molality is crucial in solution thermodynamics because it allows for accurate conversion between volume-based and mass-based properties. This is essential for correctly applying thermodynamic equations and understanding how solution properties change with concentration and temperature.

51. How does the concept of apparent molar volume relate to the difference between molarity and molality?

The apparent molar volume, which represents the volume change when adding solute to a solvent, directly affects the relationship between molarity and molality. It explains why the simple conversion between these units based on density alone is often inaccurate, especially for concentrated solutions or solutions of large molecules.

52. What is the importance of specifying temperature when reporting molar concentrations, and why is this less critical for molal concentrations?

Specifying temperature is important for molar concentrations because solution volume, and thus molarity, changes with temperature. Molal concentrations, being based on mass, are less affected by temperature changes, making temperature specification less critical but still relevant for precise work.

53. How does the concept of limiting ionic conductivity relate to the choice between molarity and molality in electrolyte solutions?

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.

54. How does the choice between molarity and molality affect the calculation of reaction quotients and equilibrium constants in non-ideal solutions?

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.

55. How does the concept of partial molar quantities relate to the difference between molarity and molality in solution thermodynamics?

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.

56. Why is understanding both molarity and molality important in the context of solution mixing and dilution processes?

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.

57. How does the choice between molarity and molality affect the interpretation of spectroscopic titration data?

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.