Different units used to express the relative amount of a substance in the mixture are called concentration units. Some important concentration units include mass percentage, volume percentage, ppm, ppb, molarity, normality, molality, etc.
Molality is the measure of the concentration of solute in a solution. It is commonly used in chemical and biological applications where the temperature and pressure of the solution can affect its density.
Molality is different from molarity. The changes in temperature and pressure affect the density of solutions, making molarity an unreliable measure of concentration. Molality, on the other hand, is a more reliable concentration unit, in this matter.
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Definition
The number of moles of solute dissolved per kilogram of the solvent is called molality. It is symbolized as “m”.
Formula
molality = moles of solute / kg of solvent
m = (mass in grams/molar mass) (1/kg of solvent)
Units
The unit of molality is mol/kg or m.
A solution having one mole of solute dissolved in one kg solvent is called one molal or 1 m. Therefore;
1 m = 1 molal
Importance of Molality
Molality is an important concentration unit when dealing with the mass (weight) of solvent.
Molality is independent of the volume of the solution or solvent
Molality is an intensive property, that is calculated from moles of solute and kg of solvent used to make up that solution. Along with the volume of the solution, molality is also independent of changes in temperature and pressure because mass is not affected by changing these parameters.
Relationship with Molarity
If the molarity and density of the solution are given, one can easily calculate the molality of the solution.
1/m = (ρ/M) – (M1/1000)
where,
- m = molality
- M = molarity
- ρ = density of solution (g/mL)
- M1 = molar mass of solute
Molality vs. Molarity
| Molarity | Molality |
| The number of moles of solute dissolved per liter of a solution | The number of moles of solute dissolved per kilogram of a solvent |
| It is symbolized as M | It is symbolized as m |
| It is dependent on changes in temperature and pressure | It is independent of changes in temperature and pressure |
| M = moles / liters of solution | m = moles / kg of solvent |
| One molar solution is a solution with one mole of solute dissolved in enough solvent to make up one liter solution | One molal solution is a solution with one mole of solute dissolved in a one kilogram solvent |
|
M = (1000ρm) / (1000+mM1) |
1/m = (ρ/M) – (M1/1000) |
|
Its unit is mol/L or mol/dm3 or M |
Its unit is mol/kg or m |
Example no.1
Calculate the molality of 3M NaCl solution whose density is 1.12 g/mL.
Solution:
Molarity = M = 3M
Density = ρ = 1.12 g/mL
Molar mass of NaCl = M1 = 58.5 g/mol
1/m = (ρ/M)-(M1/1000)
1/m = (1.12/3)-(58.5/1000)
m = 3.17 m
Example no.2
Calculate the molality of 2M HCl solution having a density of 1.03 g/mL.
Solution:
Molarity = M = 2M
Density = ρ = 1.03 g/mL
Molar mass of HCl = M1 = 36.5 g/mol
1/m = (ρ/M)-(M1/1000)
= (1.03/2)-(36.5/1000)
m = 2.09 m
Example no.3
74.5 grams of KCl is dissolved in 2 kg solvent to make a solution. Calculate the molality of the solution.
Solution:
Mass of KCl = 74.5 g
Mass of solvent = 2 kg
Molar mass of KCl = 39+35.5 = 74.5 g/mol
Moles of KCl = 74.5/74.5
= 1 mol
m = moles of solute/kg of solvent
= 1 mol / 2 kg
= 0.5 mol/kg or 0.5 m
Example no.4
Calculate the molality of 0.23 moles of NaCl in 230 g of solvent.
Solution:
Moles of NaCl = 0.23 mol
Mass of solvent = 230 g = 0.23 kg
m = moles of solute/kg of solvent
= 0.23/0.23
= 1 mol/kg or 1 m
See Atomic Mass vs. Molar Mass for more concepts.
Concepts Berg
When to use molality and when to use molarity?
Molality is used when the concentration of a solution needs to be expressed in terms of the amount of solute per unit mass of solvent. Whereas, molarity is used when the concentration of a solution needs to be expressed in terms of the amount of solute per unit volume of solution.
Molality is advantageous when studying the behavior of solutions at different temperatures, while molarity is more useful when studying the behavior of solutions at a single temperature.
What is the advantage of molarity over molality?
Molarity is used when the concentration of a solution needs to be expressed in terms of the amount of solute per unit volume of solution. Molarity is more useful when studying the behavior of solutions at a constant temperature.
What is the unit of molality?
mol/kg or molal(m) is the unit of molality.
What are molality and normality?
Molality is a unit of measurement of the amount of solute (substance dissolved in a liquid) present in a solution. It is expressed as moles of solute per kilogram of solvent. Normality is a unit of measurement of the amount of a solute present in a solution. It is expressed as the equivalent number of moles of solute per liter of solution.
How do you calculate total molality?
Total molality (m) is calculated by dividing the total number of moles of solute (n) by the total kilograms of solvent (w):
m = n/w
What is the relation between molarity and molality?
1/m = (ρ/M)-(M1/1000)
where,
- M = molarity
- ρ = density of solution (g/mL)
- M1 = molar mass of solute
Can molality be greater than 1?
Yes, molality can be greater than 1. It measures the number of moles of solute per kilogram of solvent and can be any value above 0.
How to calculate molality from molarity and density?
1/m = (ρ/M)-(M1/1000)
where,
- m = molality
- M = molarity
- ρ = density of the solution
- M1 = molar mass of solute
