Difference Between Molarity and Molality: Definitions, Examples, Formula, Relation, Key Differences, and Sample Questions

Difference Between Molarity and Molality Overview

Both molarity and molality are measurements of the concentration of a chemical solution. The major distinction between the two is one of mass versus volume. The molality explains a solute's moles in relation to the mass of a solvent, whereas the molarity describes a solute's moles in relation to the volume of a solution. The molality of a solute is the number of moles per mass of a solvent, whereas the molarity is the number of moles per volume of a solution.

What is a Mole?

A mole is the standard unit for expressing the number of atoms and molecules contained in a material. One mole is represented by 12 grams of pure carbon-12 powder.

Difference Between Molarity and Molality Definitions

What is Molarity?

The total number of moles of solute per liter of a solution is defined as its molarity. It is determined by parameters like volume, temperature, and pressure. M represents it. It's also known as molar concentration.

What is Molality?

Molality (m), also known as molal concentration, is the quantity of a material dissolved in a given mass of solvent. It is defined as the moles of a solute per kilogram of a solvent. Molality is also known as molal concentration. Molality has an advantage over molarity in that it is unaffected by changes in temperature and pressure because it is determined by mass rather than volume.

Relation Between Molarity and Molality

Below is the relation between molarity and molality

Where,

ρ is the density of the solution (mg/mL).

M₁ is the molecular weight of the solute. 

Difference Between Molarity and Molality Examples

Example of Molarity

If you dissolve table salt in water, the salt is the solute and the water is the solution. The weight of one mole of sodium chloride is 58.44 grams. A one-molar solution, abbreviated as 1M, is formed by dissolving 58.44 grams of NaCl in one liter of water.

Example of Molality

When 2.5 grams of ethanoic acid are dissolved in 75 grams of benzene.

Solution- Ethanoic acid has a molar mass of 60 g/mol.

Ethanoic acid moles = 2.5/60 = 0.0417 mol

Benzene mass in kg = 7/1000 kilogram

Ethanoic acid molality = moles of ethanoic acid/mass of benzene

= 0.0417 × 1000 / 75 

= 0.556 mol/kg

Read more about the Mole Fraction.

Difference Between Molarity and Molality Formula

Formula for Molarity

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

Formula for Molality

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

How to Calculate Molarity and Molality?

Molarity Calculator: Here's a useful calculator for calculating the molarity of a solution.

Molality Calculator: A molality calculator may be found here.

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Difference Between Molarity and Molality

The following table shows the difference between molarity and molality- 

Also read more about the- 

Key Difference Between Molarity and Molality

1. A solution's molarity is defined as a mole of solute dissolved in a liter of solution, but its molality is defined as a mole of solute dissolved in a kilogram of solution.
2. The molarity value is not regarded as accurate or exact. The value of molality, on the other hand, is more exact and precise.
3. Molality is expressed in mol/kg, whereas molarity is expressed in mol/L.
4. The molality of a solution is also indicated by "M," whereas the molality of a solution is denoted by "m."
5. The volume of a particular solution can be used to calculate the molarity of a solution. The molality of a solution, on the other hand, may be calculated using the mass of the provided solvent.
6. The value of molarity is changed by pressure changes, whereas the value of molality is unaltered.
7. Molality fluctuates with temperature fluctuations, although temperature fluctuations have no effect on the value of molality.
8. A specific molar concentration of sugar can be prepared. The weight of the sugar must first be converted to moles before adding water until 1 liter is obtained.

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Sample Questions on the Difference Between Molarity and Molality

Sample Question 1- What is the mass of a 0.449 molal KBr sample containing 2.92 kg of water?

Ans. Molality = moles solute divided by kilograms solute

0.449 mol/kg = x / 2.92 kg

x = 1.31108 mol of KBr

Moles times molar mass equals grams

(1.31108 mol) (119.0023 g/mol) = 156 g KBr

Adding them up

156 g KBr + 2920 g water = 3076 g total

Sample Question 2- Differentiate by concentration: 1M vs 1m.

Ans. 1M denotes one mole of solute in one liter of solution. We can see here that the solution is a blend of solute and solvent. 1m denotes one mole of solute in one kilogram of solution. In this case, the solution is 1 liter of solvent. This means that the amount of solvent in 1 mL of solution is greater than the amount of solvent in 1 mL of solution for the same amount of solute. As a result, the concentration of 1M solutions is higher than that of 1M solutions containing the same number of moles of solute.

Sample Question 3- What is the molality of a 138 solution of sodium sulfate in water (solvent)?

Ans. The stated weight of sodium ions = 138 grams.

Sodium has a molecular mass = 23 grams.

Number of sodium ion moles = 138/23 = 6 moles

1 kilogram of the specified solvent

Hence, molality is defined as the number of moles of sodium ions divided by the weight of the solvent in kilograms.

= 6/1

= 6 m

Points to Remember

1. The words molarity and molality are used to describe the composition and concentration of a solution.
2. The solute is the component present in tiny amounts in a solution.
3. The solvent is the most prevalent component in a solution.
4. The solution is formed by the solute and solvent combinations.
5. Molarity is defined as the number of moles of the solute per liter of solution. M represents it.
6. The number of moles of the solute per kilogram of solvent is called molality. It is represented by m.
7. The study of solution characteristics in relation to vapour pressure and temperature variations makes use of concentration represented in molarity and molality.

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