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Molality is One of the crucial characteristics of solutions. The concentration of a solute in a solution is expressed using a formula that is primarily based on the mass of the solvent. The terms “molality" and "molal concentration" are also used frequently. It is represented using the letter “m”
- The primary way a molality was defined was in relation to molarity, which is the molar concentration of a solution.
- The first time this attribute is mentioned in a publication is in a 1923 work by G. N. Lewis and M. Randall. The topic was covered in the book "Thermodynamics and the Free Energies of Chemical Substances."
Read more: Archimedes principle
Key Terms: Molality, Moles, Solvent, Solute, Solutions, Molarity
Molality Definition and Formula
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The amount of moles of solute per kilogram of solvent is known as molality (m). The following is the molality formula:
Molality (m) = moles of solute / kilograms of solvent
It is to be noted that molality, not solution mass, is employed to determine the moles in relation to solvent mass.
Additionally, there are times when the solutions include more than one solvent. When this occurs, we take into account or define the molality of the combined solvent. The units can also be stated as moles of solute per kilogramme of mixed solvent.
SI Unit of Molality
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Mole per kilogramme (mol/kg) is the SI unit for molality. For instance, a solution with a molality of 6 mol/kg is expressed as 6 molal, or 6 m.
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Benefits and drawbacks of using molality
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Molality has both benefits and drawbacks. They are:
Advantages of Molality
The advantages of molality are as follows:
- Molality can be employed as a concentration indicator.
- Molality exclusively depends on the mass of the solute and the solvent.
- Unlike in the case of solutions made volumetrically, mass is not impacted by variations in temperature or even pressure.
- The molality of one solute in a solution is not reliant on the presence of other solutes, which makes it very helpful.
Disadvantages of Molality
The disadvantages of molality are as follows:
- The lack of applicability in situations when there is no pure material in a mixture is one of its drawbacks.
- For instance, alloys or mixes like alcohol and water. Any material could serve as the solvent in this case.
Molarity versus Molality
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These two methods for expressing a chemical solution's concentration are distinct from one another. Additionally, they have diverse applications. The differences between them are as follows:
| Molality | Molarity |
|---|---|
| The quantity is the moles of solute per kilogram of solvent. | It is the number of moles of solute per kilogram of solvent. |
| Change in temperature will not affect this unit of concentration. | The unit will change with change in temperature. |
| Its unit is mol/kg. | Its unit is mol/liter. |
| Denoted by the letter (m). | Denoted by the letter (M). |
While weak aqueous solutions are the sole situation in which molality and molarity are comparable. At normal temperature, one kilogramme of water (the solvent) typically takes up one liter of space. According to the theory, a modest amount of solute will have little impact on the volume.
Mole fraction
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The mole fraction is the number of molecules in a mixture of a specific component, divided by the total number of moles in that mixture. Mole fraction is a unitless quantity. Symbol ‘X’ represents the mole fraction.
Let's say the solution includes the parts "A" and "B." The mole fraction "X" can thus be expressed as:
and
Percent by Weight
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The ratio of the solute to the mass of the solution multiplied by 100 determines the percent by weight. A solution is created when a solute dissolves in a solvent. By dividing the mass of the solution by the masses of the solute and solvent, the percent by weight calculates the concentration of each. Since the mass of the solute or solvent cannot be more than the mass of the solution, its value is never greater than 100%. Given that it gives the mass ratio, it is a quantity without units.
Formula
Per cent by weight = gram of solute/100 g of solution
Things to Remember
- Molarity and Molality are terms used to express the composition and concentration of a solution.
- In a solution, the solute is the component present in small quantities.
- In a solution, the solvent is the component present in large quantities.
- The solute and solvent mixture forms the solution.
- Molality is defined as the number of moles of the solute per kg of the solvent. It is denoted by m.
Sample Questions
Ques. Describe molality (1 mark)
Ans. Molality is a term used to describe concentration.
Molality = number of moles of solute / kilogramme of solvent
Ques. What is the molality unit in the SI? (1 mark)
Ans. Molality is measured in SI units of mol/kg.
Ques. Does molality depend on temperature? (1 mark)
Ans. No. Molality is not temperature dependent.
Ques. What are molality and molarity? (2 marks)
Ans. Molarity, also referred to as molar concentration, is the quantity of a material expressed as moles per liter of solution. Solutions marked with a molar concentration have a capital M. One mole of solvent per liter is present in a 1.0 M solution. The number of solvent moles per kilogramme is known as molality.
Ques. What is the purpose of Molality? (1 mark)
Ans. Since the mass of the solute and solvent in the solution remains constant, molality is the ideal method for transmitting concentration.
Ques. What benefit does Molarity have over Molality? (2 marks)
Ans. Practically speaking, laboratory testing is preferable. Molarity is less dependable than molality, despite temperature changes affecting volume in molarity rather than molality. Greater than mistakes from a molality balance are molarity error sources from volume estimation.
Ques. How may molarity be used? (1 mark)
Ans. One of the most popular measures for measuring concentration is molarity (M), which stands for the quantity of moles of solute per liter of solution (moles/L).
Ques. Which do you prefer, molarity or molality? (2 Marks)
Ans. The molarity of a solution is the amount of solute in moles per liter of the solution. The amount of moles of solute in a kilogramme of solvent is known as molality. This demonstrates the relationship between molarity and solution volume. Thus, a change in the system's temperature may have an impact on the solution's molarity. The mass of the solution determines molality, which has no bearing on a temperature change. Because of its independence from temperature, molality is a better option for expressing solution concentrations.
Ques. Determine the molality of 75 g of benzene and 2.5 g of ethanoic acid (CH3COOH). (5 marks)
Ans. Molar mass of C2H4O2 =12 x 2 + 1 x 4 + 16 x 2 = 60 g mol–1
Moles of C2H4O2 = (2.5g)/(60g\mol-1)=0.0417 mol
Mass of benzene in kg = (75 g)/(1000 g\ kg–1) = 75 x 10–3 kg
Molality of C2H4O2 =(0.0417 mol) / (75x10-3 kg)=0.556 mol kg-1
Ques. Determine the molality of the mixture of 16.5 g naphthalene (C10H8) and 54.3 g benzene (C6H6). (3 marks)
Ans. Molality = kilos of benzene/moles of naphthalene
(2.37 m) = (16.5 g / 128.1732 g/mol) / 0.0543 kilogram
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