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Difference Between Molarity and Molality

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  • Last Updated : 23 Jun, 2022

Molarity and Molality are the terms 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. The concentration of a chemical solution is measured by both molarity and molality. The major difference between the two is one of mass vs. volume.
The molality refers to the number of moles of a solute per mass of a solvent, whereas the molarity refers to the number of moles of a solute per volume of a solution.
Continue reading for more information about molarity and molality, including definitions, calculations, and a comparison between the two concepts.

 

What is Molarity?

The molarity of a solution is defined as the total number of moles of solute per liter of a solution. It depends on some the factors like volume, temperature, and pressure. It is denoted by M. It is also called Molar Concentration.

The formula to calculate the Molarity, 

M = Number of moles of the solute/Volume of the solution in litres

  • The units of molarity are M or mol/L. 
  • A 1 M solution is said to be “one molar.”

Molarity is a unit of measurement that may be used to determine the concentration of a diluted substance. When perfect accuracy is not necessary, molarity can be used. Because it is a volumetric measurement, it is influenced by variations in temperature, hence it may not be acceptable to use in some instances.

What is 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 denoted by m. It is also called Molal Concentration. 

The formula to calculate the Molality, 

m = Number of moles of the solute/Mass of the solution in litres

The units of molality are m or mol/kg.

Molality is also referred to as molal concentration.

Because it is determined based on mass rather than volume, molality has an advantage over molarity in that it is unaffected by changes in temperature and pressure. Because the mass of the solvent is not changed by temperature in the same way that a substance’s volume is, molality is a more precise measure of concentration than molarity.

Difference between Molarity and Molality

Molarity

Molality

The molarity of a solution is defined as the total number of moles of solute per liter of a solution. The molality of a solution is defined as the number of moles of a solute per kilogram of solvent.

The formula to calculate the Molarity, 

M = Number of moles of the solute/volume of the solution in liters.

The formula to calculate the Molality, 

m = Number of moles of the solute/Mass of the solution in litres.

The units of molarity are M or mol/L.  The units of molality are m or mol/kg.
It is denoted by M. It is denoted by m.
It depends on the volume, temperature, and pressure of the solvent. It doesn’t depend on the volume, temperature, and pressure of the solvent.
It does not depend on the mass of the solvent It depends on the mass of the solvent.
It is also called Molar Concentration. It is also called Molal Concentration.
It increases with an increase in Temperature It does not change with the changes in the temperature.
It is not reliable. It is reliable.
It may be accurate or inaccurate. It is very accurate.

Sample Questions

Question 1: Write the relationship between molarity and molality

Answer:

Molarity of a solution is defined as the total number of moles of solute per litre of a solution. It depends on some of the factors like volume, temperature and pressure. It is denoted by M. It is also called as Molar Concentration.

Molality of a solution is defined as the number moles of a solute per kilogram of solvent. It depends on the mass of the solvent. It is denoted by m. It is also called as Molal Concentration. 

Relationship between Molarity and Molality:

m = (1000 × M)/(1000 × (d – M) × Mb)

where, 

  • m = molality,
  • M = molarity
  • d = density of solvent.
  • Mb = Molecular mass of the solution.

Question 2: Calculate the molarity of a solution if it has a molality of 1.45 mol/kg and the density of solvent is 0.15 kg/m3.

Answer: 

Given, molality m = 1.45 mol/kg 

Density d = 0.15 kg/m3.

We know that from molality formula,

m = number of moles of the solute/mass of solvent in kgs  

Density d = weight/volume

Weight = density  ×  volume

m = no of moles of solute/ (d × volume)

m = M/d

M = d  ×  m

= 0.15 × 1.45

= 0.2175 mol/L.

Therefore, the molarity of solution is 0.2175 mol/L.

Question 3: The density of a solution is 1.25 g/mL which is prepared by dissolving 180 g of urea in 1200 g if water. Find the molarity of the solution. (Molecular mass of urea = 60gm/mol).

Answer: 

Given that,

Molecular mass of urea = 60gm/mol.

Weight = 180gm

No.of moles of urea = 180/60 = 3 moles.

Volume of the solution = mass of the solution/ density

Mass of the solution = 1200+180 = 1380gm

Volume = 1380(gm)/1.25(gm/mL)

= 1104mL = 1.104L

Molarity M = no.of moles of urea/volume

M = 3/1.104

= 2.72 mol/L.

Therefore, the molarity of the solution is 2.72 mol/L.

Question 4: If 150 gm of KCl is dissolved in water make a solution of 50 L. Find the molarity of a solution. (Molar mass of KCl is 74.5 gm/mol).

Answer: 

In 150gm of KCl

No of moles = 150/74.5

= 2.01 moles.

Formula of Molarity,

M= Number of moles of the solute/  volume of the solution in litres

Molarity = 2.01/50 = 0.040 mol/L.

Therefore, the molarity of the solution is 0.040 mol/L.

Question 5: What is the molality of the solution prepared from 48gm of toluene (C7H8) dissolved in 634 gm of benzene (C6H6). (Molar mass of toluene is 92gm/mol.)

Answer: 

Given 

Weight of toluene = 48 g

Molar mass of toluene =  92gm/mol.

No. of moles toluene = 48/92

= 0.522 moles

Formula Molality 

m = Number of moles of the solute/mass of the solvent in kgs

= 0.522/0.634

= 0.823 mol/kg.

Therefore, the molality of solution is 0.823 mol/kg

Question 6: Calculate the molality of 150 g of KBr dissolved in 175 g of pure water.

Answer:

Given weight of KBr = 150 g

Weight of pure water = 175 g

No of moles of KBr = 150/119

= 1.26 moles

Formula of molality 

M= Number of moles of the solute/ Mass of the solvent in kgs

= 1.26/0.175

= 7.21 mol/kg.

Therefore, the molality of solution is 7.21 mol/ kg.

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