Molality - Definition, Formula, Examples, FAQs

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What is Molality and its Formula?

Molality definition is the number of moles of solute present in 1000 gm of solvent. Unlike molarity units, Molality does not change with temperature because size is affected by changes in temperature. The important thing to note here is that we use the weight of 'Solvent' and not 'Solution' to calculate equity, which can be an easy mistake to make when solving problems. e.g., solution 1 of molal NaCl means that one molecule of NaCl was dissolved in 1 kg of water Statistically, beast (m) = number of bodies weight (n) weight of solvent in Kg

This Story also Contains

What is Molality and its Formula?
Definition of Molality
Definition of Molarity
Molarity vs Molality
What is molality?
SI Unit of Molality
Advantages and disadvantages of using Molality
Molarity and molality formula:
Molal solution meaning

Example: Calculate the amount of solution prepared from 29.22 grams of NaCl in 2.00 kg of water.

Solution:

Solute = 29.22 gm of NaCl

Solution = 2.00 Kg of water

Molar solute weight (NaCl) = 58.44 g / mol

number of moles of solute = 29.2258.44 = 0.5moles

beast (m) = number of bodies weight (n) weight of solvent in Kg

m = 0.52 = 0.25moles / kg

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Question: 4 g cube of sugar (Sucrose: C₁₂H₂₂O₁₁) is poured into a 350 ml teacup of 80 ° C water. What is the sensitivity of a sugar solution?

Given: Water volume at 80 ° = 0.975 g / ml

Options:

0.034 mol / kg
0.075 mol / kg
0.089 mol / kg
0.010 mol / kg

Answer: (a)

Solution:

Solute weight is 4 g of C₁₂H₂₂O₁₁ molar mass of C₁₂H₂₂O₁₁ = 342 g / mol divided this number by the sample size of molecules of ₁₂H₂₂O₁₁ = 4 g / (342 g / mol) = 0.0117 mol

Now,

size = weight / volume = density x volume mass = 0.975 g / ml x 350 ml mass = 341.25 g mass = 0.341 kg

Therefore,

Molality = moles of solute / mass of solvent (kg)

Molality = 0.0117 mol / 0.341 kg

Molality = 0.034 mol / kg

Answer: The maximum concentration of sugar solution is 0.034 mol / kg.

Both cohesion and quantity are the means of concentration of a chemical solution. The main difference between the two falls in weight compared to volume.

Molality solute moles in relation to the solvent size, while the molecule is related to solute molecules in terms of the solution volume.

Definition of Molality

"Molality is the amount of substance dissolved in a particular solvent concentration".

They are described as solute moles in kilograms of solvent.

Morality formula and units

Molality units are m or mol / kg.

Molality Statistics

m = moles solute / kilograms only

Definition of Molarity

Molarity (M) is the number of objects in a certain volume of solution.

" Molarity units is solute moles per litre of solution".

Molarity is also known as the integration of a molar solution.

Molarity vs Molality

An important difference between the quantity and the solvent is the difference between the solution and the solvent.

Molarity is the ratio of moles of solute to a complete litre of solution.

Molality, on the other hand, is the ratio of solute molecules to kilograms of solvent. Note that the weight used in the denominator is that of the solvent only, not the solute and solvent.

What is molality?

Molality definition chemistry is "the total moles of a solute contained in a kilogram of solvent." Molality is also known as molal concentration. It is a solute concentration solution. The solution is made up of two elements: solute and only. There are many ways to express solutions such as molarity molality normality, formality, volume percentage, weight percentage and half per million. This term requires the calculation of the solvent size and solute molecules.

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SI Unit of Molality

SI unit of Molality are moles per kilogram (mol / kg).

For example, the solution given for a molar 6 mol / kg is called 6 molal or 6 m.

Advantages and disadvantages of using Molality

What this means is that these substances are not affected by changes in temperature or pressure as in a voluntary solution. Second, it is very useful because the solution for one solute is independent of the presence of other solutes.

With the advent of evil, it does not work in situations where there is nothing pure in the organization.

For example, a mixture of water and alcohol, alloys, etc. Here, any of that element can be considered a solvent.

Molality vs Molarity

Molality and Molarity are words that are often confusing, and some students even use them differently.

However, these are two different steps to represent the concentration of a chemical solution.

At present, the only thing that is the same as unity is when we talk about weak and powerful solutions. Generally, if we take one kilogram of water (solvent) it will take a volume of 1 liter at room temperature. Depending on the goal, a small amount of solute will have a small impact on volume.

Molality is used to express the concentration of solutions related to temperature changes and vapour pressure.

Molality is also used in determining the boiling or melting point and when working with composite structures. In terms of calculation, Molality can be easily determined by knowing the solute and solvent size of the solution. Moreover, the concentration of the same solution or saturation of the body remains constant.

Molarity and molality formula:

Molality formula

Molality (M) = Number of solute moles / M mass of solvent per kgs

Molality (M) = g × 1000W × m

Molarity formula and units

The unit of molarity is M or mol / L.

The solution of 1 M is said to be "one molar."

Molarity calculation

Solution for M = moles solute / litres

Also Read:

Molal solution meaning

A molal solution is a solution containing a mass of 1 kilogram molecule of solvent. It is the strength or concentration of the solution, especially the amount of soluble matter in a given volume of solvent. It is the concentration of the solution expressed in moles or Molality (m). Volume is not a part of any Molality equation; Molality is independent of temperature. Using honours rather than reducing lab testing keeps results a close distance. In the field of electrochemistry and metal corrosion, equilibrium and solidarity are used as targets.

Examples of Molality

Calculate the quantity of solution when 0.5 grams of toluene (C₇H₈) dissolved in 225 grams of Benzene (C₆H₆). Calculate the solute moles provided.

Solution:

Toluene: molecular weight = toluene = 7 × 12 + 1 × 8 = 92 grams / mole

The formula is used:

Toluene mole = Grammar mass Weighed molecules = 5.0 grams 92 grams / mole = 0.054 mole.

Therefore, toluene mole is 0.054 mol.

Now count only a pound.

225 grams of Benzene /1000 = 0.225 kilograms

As a final step, calculate the decay using a formula.

Molality (m) = Toluene Moles Maximum Benzene per gram

Molality (m) = 0.054 moles0.225 kg Molality (m) = 0.24 m

Also check-

NCERT Chemistry Notes:

Frequently Asked Questions (FAQs)

1. Define molarity and molality?

Molarity is expressed as the number of moles of matter in each litre of solution, also known as molar concentration. Capital M displays solutions with molar filtering labels. The 1.0 M solution contains 1 molecule of solvent per litre of solution. Molality is the number of moles only per kilogram.

2. What is molality definition and formula?

Molality is the property of the solution and is defined as the number of moving moles per kilogram. The unit of Molality's SI is mol / kg. A solution with a molar / kg Molality of 3 is often defined as “3 molal” or “3 m.” However, it is now preferable to follow the unit SI, mol / kg or similar SI unit.

3. What is molality in chemistry and why should we use Molality?

Molality is a preferred method of conveying concentration because the solute and solvent solution does not change.

4. What is the benefit of harmony over Molality?

Practical testing in the laboratory is best. Or because of differences in temperature that affect volume in quantity and not coherence, it cannot be as reliable. Molarity error sources from volume measurement are also larger than errors from Molality balance.

5. How useful is molarity?

Molarity (M) is one of the most common units used to measure a solution, representing the number of solute moles per litre of solution (moles / Litre).

6. Why is molality preferred over molarity in some situations?

Molality is preferred in situations where temperature changes are involved because it doesn't depend on volume, which can change with temperature. This makes molality useful in studying colligative properties, freezing point depression, and boiling point elevation.

7. Can molality be used for gases?

Molality is typically used for solutions with liquid solvents. For gases, other concentration units like partial pressure or mole fraction are more commonly used.

8. How does molality relate to colligative properties?

Molality is directly related to colligative properties, which depend on the number of dissolved particles rather than their nature. As molality increases, effects on freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure become more pronounced.

9. Why is water often used as the solvent when discussing molality?

Water is commonly used as a solvent when discussing molality because it's a universal solvent, has well-known properties, and is frequently used in laboratory and real-world applications. However, molality can be calculated for any solvent.

10. How does molality affect the freezing point of a solution?

Increasing the molality of a solution lowers its freezing point. This relationship is described by the equation ΔTf = Kf × m, where ΔTf is the freezing point depression, Kf is the freezing point depression constant, and m is the molality.

11. Can molality be used to describe the concentration of ionic compounds?

Yes, molality can be used for ionic compounds. However, when considering colligative properties, the van 't Hoff factor must be taken into account to reflect the increased number of particles due to dissociation.

12. What's the relationship between molality and mole fraction?

Mole fraction and molality are related, but not directly proportional. For dilute solutions, the mole fraction of the solute is approximately equal to the molality multiplied by the molar mass of the solvent.

13. How does molality affect the partial molar properties of a solution?

Partial molar properties, which describe how a property of a solution changes with the addition of a component, are influenced by molality. As molality increases, partial molar properties often deviate from their ideal values due to increased interactions.

14. How does molality relate to the concept of osmolality?

Osmolality is closely related to molality. It measures the concentration of osmotically active particles in a solution, expressed as osmoles per kilogram of solvent. For ideal, dilute solutions of non-electrolytes, osmolality is equal to molality.

15. How does molality relate to the concept of colligative properties in non-aqueous solutions?

Molality is particularly useful for describing colligative properties in non-aqueous solutions because it's independent of temperature and thermal expansion. This makes it easier to predict properties like freezing point depression in organic solvents.

16. Can molality be used in electrochemistry?

Yes, molality can be used in electrochemistry, especially when dealing with non-aqueous solutions or situations where temperature changes are significant. However, molarity is more commonly used due to its direct relationship with volume.

17. Can molality be used in chromatography?

While molality can be used to express concentrations in chromatography, it's not commonly used. Mobile phase compositions in chromatography are typically expressed in volume percentages or molarity due to the importance of volume in flow systems.

18. Can molality be used in buffer solutions?

Yes, molality can be used to describe concentrations in buffer solutions. However, molarity is more commonly used because buffer capacity is typically expressed per unit volume of solution.

19. Can molality be greater than 1?

Yes, molality can be greater than 1. This occurs when there is more than one mole of solute per kilogram of solvent. For example, a solution with 2 moles of solute in 1 kg of solvent would have a molality of 2 m.

20. How does the molecular weight of the solute affect molality?

The molecular weight of the solute doesn't directly affect molality. However, it does influence how much solute (by mass) is needed to achieve a certain molality. A higher molecular weight means more grams of solute are needed for the same number of moles.

21. Does adding more solvent change the molality of a solution?

Yes, adding more solvent decreases the molality of a solution. This is because molality is inversely proportional to the mass of solvent. More solvent means a lower concentration of solute per kg of solvent.

22. How does molality relate to the concept of ideal solutions?

In ideal solutions, molality can be used to predict colligative properties accurately. However, in non-ideal solutions, deviations occur due to interactions between solute and solvent particles, requiring corrections to molality-based calculations.

23. Can you have a negative molality?

No, you cannot have a negative molality. Molality is always positive or zero, as it's impossible to have a negative number of moles or a negative mass of solvent.

24. How do you calculate molality?

Molality is calculated by dividing the number of moles of solute by the mass of solvent in kilograms. The formula is: molality = moles of solute / kg of solvent.

25. How does molality change if you double the amount of solute?

If you double the amount of solute while keeping the solvent mass constant, the molality will double. This is because molality is directly proportional to the number of moles of solute.

26. Can you convert between molality and molarity?

Yes, you can convert between molality and molarity, but you need to know the density of the solution and the molecular weight of the solute. The conversion isn't straightforward because molality uses mass of solvent while molarity uses volume of solution.

27. What's the relationship between osmotic pressure and molality?

Osmotic pressure is directly proportional to molality for dilute solutions. The van 't Hoff equation relates osmotic pressure (π) to molality (m): π = MRT, where M is molality, R is the gas constant, and T is temperature.

28. Why is molality used in antifreeze solutions?

Molality is used in antifreeze solutions because it directly relates to freezing point depression, which is temperature-independent. This allows for accurate prediction of the solution's freezing point regardless of temperature changes.

29. What is molality and how does it differ from molarity?

Molality is a concentration unit that measures the number of moles of solute per kilogram of solvent. It differs from molarity in that molality uses mass of solvent, while molarity uses volume of solution. Molality is temperature-independent, making it useful for certain applications.

30. What's the difference between mass percent and molality?

Mass percent expresses concentration as grams of solute per 100 grams of solution, while molality is moles of solute per kilogram of solvent. Mass percent doesn't consider the molecular nature of the solute, whereas molality does.

31. What's the difference between molality and normality?

Molality measures moles of solute per kg of solvent, while normality measures equivalents of solute per liter of solution. Normality takes into account the reactive capacity of the solute, making it useful in acid-base chemistry.

32. How does molality relate to Raoult's law?

Raoult's law can be expressed in terms of molality for dilute solutions. The vapor pressure lowering is proportional to the molality of the solution, allowing for predictions of vapor pressure based on concentration.

33. Can molality be used to describe the concentration of suspensions?

Molality is not typically used for suspensions because it's defined for solutions where the solute is completely dissolved. For suspensions, other measures like mass percent or volume percent are more appropriate.

34. What are the units of molality?

The units of molality are moles per kilogram, often written as mol/kg or m (not to be confused with molar, M).

35. What's the relationship between molality and the van 't Hoff factor?

The van 't Hoff factor (i) is used to correct molality when calculating colligative properties of electrolyte solutions. It's multiplied by the molality to account for the increased number of particles due to dissociation: effective molality = i × molality.

36. How does molality relate to the concept of osmotic coefficient?

The osmotic coefficient is a factor used to correct the ideal osmotic pressure calculated from molality. It accounts for deviations from ideal behavior in real solutions, especially for electrolytes at higher concentrations.

37. Can molality be used for non-electrolyte solutions?

Yes, molality can be used for both electrolyte and non-electrolyte solutions. However, for electrolyte solutions, the van 't Hoff factor must be considered when calculating colligative properties.

38. How does molality relate to Henry's law?

Henry's law describes the solubility of gases in liquids and is often expressed in terms of molality. The law states that the amount of dissolved gas is proportional to its partial pressure above the liquid, with the proportionality constant often given in molality units.

39. How does molality affect the boiling point of a solution?

Increasing the molality of a solution raises its boiling point. This relationship is described by the equation ΔTb = Kb × m, where ΔTb is the boiling point elevation, Kb is the boiling point elevation constant, and m is the molality.

40. What's the molal freezing point depression constant?

The molal freezing point depression constant (Kf) is a property of the solvent that relates the molality of a solution to its freezing point depression. For water, Kf is 1.86 °C·kg/mol.

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

In non-ideal solutions, the activity of a solute replaces molality in thermodynamic equations. The activity coefficient relates the actual behavior of a solution to its ideal behavior, which is based on molality.

42. How does molality affect the vapor pressure of a solution?

Increasing the molality of a solution decreases its vapor pressure. This relationship is described by Raoult's law, which states that the vapor pressure of the solution is proportional to the mole fraction of the solvent.

43. How does molality affect the chemical potential of a solution?

Increasing the molality of a solution decreases its chemical potential. This relationship is important in understanding osmosis, where water moves from regions of higher chemical potential (lower molality) to regions of lower chemical potential (higher molality).

44. How does molality affect the entropy of a solution?

Increasing the molality of a solution generally increases its entropy. This is because adding more solute particles increases the number of possible arrangements in the system, leading to greater disorder.

45. How does molality relate to the concept of fugacity?

Fugacity, which is a measure of the tendency of a substance to escape from a phase, can be related to molality through activity coefficients. In non-ideal solutions, the fugacity of a component is related to its molality and activity coefficient.

46. What's the difference between ideal and non-ideal solutions in terms of molality?

In ideal solutions, properties like vapor pressure follow Raoult's law exactly based on molality. In non-ideal solutions, deviations occur due to interactions between particles, requiring the use of activity coefficients to correct molality-based calculations.

47. What's the relationship between molality and the Gibbs-Duhem equation?

The Gibbs-Duhem equation relates changes in chemical potential of components in a system. When expressed in terms of molality, it provides a way to understand how changes in the concentration of one component affect the others in a multicomponent system.

48. How does molality affect the surface tension of a solution?

Increasing the molality of a solution generally decreases its surface tension. This is because solute particles at the surface disrupt the cohesive forces between solvent molecules, reducing the surface tension.

49. What's the relationship between molality and the Debye-Hückel theory?

The Debye-Hückel theory, which describes the behavior of strong electrolytes, uses ionic strength, which is related to molality. For dilute solutions, ionic strength is proportional to molality, allowing the theory to predict activity coefficients based on concentration.

50. How does molality affect the viscosity of a solution?

Increasing the molality of a solution generally increases its viscosity. This is because the added solute particles interfere with the flow of the solvent molecules, increasing internal friction and resistance to flow.

51. Can molality be used to describe the concentration of colloids?

Molality is not typically used for colloids because they are not true solutions. Colloids are better described by other measures like mass concentration or particle number concentration.

52. How does molality relate to the concept of thermodynamic activity?

Thermodynamic activity is related to molality through activity coefficients. In ideal solutions, activity equals molality, but in non-ideal solutions, activity coefficients are used to correct for deviations from ideal behavior.

53. How does molality affect the refractive index of a solution?

Increasing the molality of a solution generally increases its refractive index. This is because the added solute particles change the way light travels through the solution, typically slowing it down and increasing the refractive index.

54. What's the relationship between molality and the Nernst equation?

The Nernst equation, which relates the reduction potential of an electrochemical reaction to the standard electrode potential, can use molality instead of activity for dilute solutions. This is useful in non-aqueous systems or when temperature changes are significant.

55. How does molality affect the solubility of gases in liquids?

Increasing the molality of a solution generally decreases the solubility of gases. This is known as the salting-out effect, where the added solute particles reduce the ability of the solvent to dissolve gases by occupying space and interacting with solvent molecules.