Molality-Definition, Formula, SI Unit, Symbol, Calculation & Examples

Molality: In Chemistry, Molality is one of the most important properties of solutions. The concentration of a solution is defined as the amount of one substance that is present when it is mixed with another material. The concentration of a solution can be expressed in a variety of ways, including molarity, molality, normalcy, formality, volume percentage, weight percentage, and part per million. In this topic, we will study the molality definition, formula, SI unit, and so on

What is Molality?

Molality is a measure of the concentration of a solute in a solution in terms of the quantity of substance in a given amount of solvent mass. Molality is used to express the concentration of a solute in a solution and is mostly determined by the solvent’s mass.

Molality Definition

Molality is a highly accurate and dependable indicator of concentration. The number of moles of a solute per mass of a solvent is referred to as molarity, whereas the number of moles of a solute per volume of a solution is referred to as molality. Molality is also known as molal concentration. The letter “m” is typically used to represent the molality of the solution. Molality is determined by the mass of the solvent as a result,  is unaffected by temperature fluctuations.

Molality formula

The Molality Formula is simply the ratio of moles of solute to kilograms of solvent. The amount of moles per kilogram of solvent is defined as molality (m). it should be noted that molality is used to calculate moles in relation to the mass of the solvent, not the mass of the solution. The formula for calculating molality is given below.

Molality Formula=

Molality Unit

According to the definition of molality, Molality is the ratio of moles of solute to solvent weight. So the molality unit measured in the SI system is moles per kilogram (mol/kg). For example, a solution with a molality of 9 mol/kg is written as 9 molal or 9 m.

Molality SI Unit = Moles per kilogram (mol/kg)

Molality Symbol

‘Molality is defined as the number of moles of solute dissolved in one kilogram of solvent. It is symbolized by a small’ m,’
For example, 5m denotes a solution with a concentration of 2 molal (2 moles of solute/kg solvent).
It must be noted that The letter ‘M’ stands for molarity. For example, 2 M denotes a solution with a concentration of 2 molars (2 moles of soute/L solution). ‘mM’ stands for milli-molar concentration,

How to Calculate Molality?

Molality is a method of calculating solution concentration in terms of solvent weight. Using the procedures outlined here, you can quickly calculate the molality of any solution.

Step 1: Separately mark the mass of the solute and the bulk of the solvent (in kg).

Step 2: Using the formula moles = mass / molecular mass, calculate the moles of solute.

Step 3: In the formula, substitute the estimated values.

Molality Formula=

Step 4: Simplify the formula used in step 3 to obtain the needed molality, with moles/kg as the unit.

Molality and Molarity Relation

Molality was originally defined as a relationship to molarity, which is defined as the molar concentration of a solution. The concentration of a solution is expressed in terms of molality. Molarity, on the other hand, is defined as the number of moles of the solute in one liter of solution. The article goes on to explain the relationship between molarity and molality.

Let the mass of the provided solute be w, the volume of the solution be v, the molality be m, the solute’s molar mass be m’, the molarity be M, and the solvent’s weight 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)

Hence, the above equation is the molality and mole fraction relation

Molality vs Molarity

Molality and molarity are frequently mistaken terms, and some students even use them interchangeably. However, molarity and molality are two different metrics of a chemical solution’s concentration. The difference between molality and molarity will be discussed further below.

Molality of Pure water

One of the most important parameters in calculating solution concentration is the molality of the solution. The number of moles of the solute and the mass of the solvent in kg can be used to calculate the molality of the solution. The density of pure water can be used to compute the number of moles of water. let’s find out the molality of pure water by following a step-by-step method

Step 1: Determine the mass of water:
The density of pure water is 0.9970749 kg/litre.
As a result, the mass in 1lt of water will be determined as follows:
=density x volume
=0.9970749 kg/lt x 1lt
= 0.9970749 kg

Step 2: Determine the number of moles in pure water:
Pure water has a molecular mass of 18.0148 g mol-1 = 0.0180148 kg mol-1
The number of moles of pure water can be estimated as
= molecular mass of water/ mass of pure water (Kg)
= 0.9970749 kg /0.0180148 kg mol-1
= 55.348

Step 3: The molality formula is given as the amount of moles of solute dissolved per kilogramme of solvent is referred to as molality.
molality Formula (m) = Moles of the solute/mass of the solvent (Kg)

Step 4: Calculating the molality of pure water:
So, the molality of pure water will be (M) = 348 / 0.9970749 = 55.510 m
As a result, the molality of pure water can be computed using the technique described above, which gets 55.510 m.

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