Molality is the term used to express the concentration of the solution. The concentration of a solution is defined as the measure of the quantity of one substance when it is mixed with another substance.
Let’s learn about Molality in detail, including its definition, SI unit formula, and solved examples.
Definition of Molality
The molality of a solution is defined as the number of moles of a solute per kilogram of solvent. It depends on the mass of the solvent. It is also referred to as Molal Concentration.
It is denoted by the letter m.
Molality SI Unit
Molality is defined as the ratio of moles of solute to the weight of solvent in k. Thu, the unit of molality is mol/kg.
Molality Formula
The formula to calculate the Molality of any solution is,
m = (Number of Moles of Solute) / (Mass of Solvent in kg)
Difference between Molality and Molarity
The concentration of a chemical solution is measured by both molarity and molality. The difference between Molarity and Molality is given below.
| Molality is the ratio of the number of moles of solute to the weight of solvent in kg. |
It is the number of moles of solute per kilogram of solvent. |
| It is denoted by the letter (m) |
It is denoted by the letter (M) |
| It is not affected by the change in temperature. |
It is affected by the change in temperature. |
| Unit of molality is mol/kg |
Unit of molarity is mol/litre |
Properties of Molality
Molality is the way of expressing the concentration of the solution. The various properties of molality are discussed below in the article,
- Molality depends on the mass of the solvent.
- Molality is independent of changes in temperature.
- Molality is the measure of concentration which is very accurate and reliable.
Advantage of Molality
Molality is the way of finding the concentration of the solution. In molality, concentration is dependent on the weight of the solvent not on the volume of the solution. So the advantages of using molality are,
- Molality of any solution does not change with the change in temperature and pressure.
- For a solution, the molality of one solute is independent of the addition of other solutes. It does not change with the addition of a newer solute.
- Molality is a more precise measure of measuring concentration than molarity.
Disadvantage of Molality
There are very few disadvantages to using molality some of the disadvantages
It is not applicable when there is no pure substance in a mixture. For example, in mixtures such as water and alcohol or alloys in such examples, no substance can be considered a solvent.
How to Calculate Molality?
Molality is the way of finding the concentration of the solution in terms of the weight of the solvent. The molality of any solution can easily be calculated using the steps given below,
Step 1: Mark the mass of the solute and the mass of the solvent (in kg) individually.
Step 2: Calculate the moles of solute using the formula :
moles = mass / molecular mass.
Step 3: Substitute the calculated values in the formula :
Molality = [moles of solute] / [mass of solvent (in kg)]
Step 4: Simplify the formula used in step 3 to get the required molality and the unit is moles/kg.
Relation between Molarity and Molality
Let the mass of the given solute be w, the volume of the solution be v, the molality be m, the molar mass of the solute be m’, the Molarity be M, the weight of the solvent be w’
Then Molarity(M) is
M = (w / m) × (1000 / v) …(1)
Now, Molality(m) is
m = (w / m’) × (1000 / w’) …(2)
Density (d) = (w + w’) / volume …(a)
from eq (1) we get,
v = (w × 1000) / (m × M) …(3)
from eq (2) we get,
w’ = (w × 1000) / (m × m’) …(4)
Now,
w + w’ = w + (w × 1000) / (m × m’)
w + w’ = (w × 1000 / m’) × (m’ /1000 + 1 / m) …(5)
dividing eq (5) by eq (3) we get, and using eq (a)
d / M = m’ / 1000 + 1 / m
1 / m = (d / M) – (m’ / 1000)
Relation between molality and mole fractionlved
Examples on Molality
Let’s solve some questions on Molality.
Example 1: Calculate the molality of 200 g of NaOH dissolved in 150 g of pure water.
Solution:
Given,
Weight of NaOH= 200 g
Weight of pure water = 150 g
We, know that molar mass of NaOH is 40 g/mol.
No of moles of NaOH = 200/40
= 5 moles
Formula of molality
M = (Number of moles of solute) / (Mass of solvent in kgs)
= 5 / 0.50
= 33.3 mol/kg
Therefore, the molality of solution is 33.3 mol/ kg.
Example 2: Calculate the molality of 1000 g of H2SO4 dissolved in 5 kg of water.
Answer:
Given
Weight of H2SO4 = 1000 g
Weight of water = 5 kg
We know that molar mass of H2SO4 is 98.08 g/mol.
No of moles of H2SO4 = 1000/98.08
= 10.2 moles
Formula of molality
M = (Number of moles of solute) / (Mass of solvent in kgs)
= 10.2/5
= 2.04 mol/kg.
Therefore, the molality of solution is 2.04 mol/ kg.
Example 3: Calculate the molality of 136 g of HCl dissolved in 25 g of water.
Answer:
Given
Weight of HCl = 136 g.
Weight of Water = 25 g.
We know that molar mass of HCl is 36.46 g/mol.
No of moles of HCl = 136/36.46
= 3.73 moles
Formula of molality
M = (Number of moles of solute)/ (Mass of solvent in kgs)
= 3.73/0.025
= 149.2 mol/kg.
Therefore, the molality of solution is 149.2 mol/ kg.
Frequently Asked Questions on Molality
What is Molality?
Molality is a way of expressing the concentration of any solution it is defined as:
Molality = Moles of Solute / Mass of solvent(in kg)
What is SI unit of Molality?
The SI unit for measuring the molality of the solution is mol/kg.
What is the difference between Molarity and Molality?
Molarity is the ratio of moles of solute to the volume of solution in litre whereas, molality is the ratio of moles of solute to the mass of solvent in kg.
Is Molality temperature dependent?
No, the molality of any solution is not temperature dependent i.e. it does not change with the change in temperature.
What is Molality used for?
Molality is used for expressing the concentration of the solution in terms of the weight of the solvent.
Why do we use Molarity over Molality?
Molarity is dependent on temperature and molality is independent of temperature. In general, we need a concentration unit which changes according to temperature. Thus, molarity is preferred over molality.
How to convert Molarity to Molality?
Molarity can easily be converted to molality using the formula,
m = (1000 × M)/(1000 × (d – M) × Mb)
where,
m = molality,
M = molarity
d = density of solvent.
Mb = Molecular mass of the solution