Short Answer

Molality is different from molarity mainly in how they measure concentration. Molality (m) is the number of moles of solute per kilogram of solvent, while molarity (M) is the number of moles of solute per litre of solution. Because molality uses mass, it does not change with temperature.

Molarity changes with temperature because it depends on volume, which expands or contracts. Thus, molality is used in temperature-sensitive calculations, while molarity is commonly used in general laboratory work and solution preparation.

Detailed Explanation

Difference Between Molality and Molarity

Molality and molarity are two important concentration units in chemistry. Although both describe how much solute is present in a solution, they use different measurements and are used for different purposes. Understanding the difference between them is essential for solving problems in solution chemistry, physical chemistry, and laboratory work.

Molality focuses on the mass of the solvent, while molarity depends on the solution’s volume. This basic difference leads to several important distinctions in how each unit behaves, especially when temperature changes.

1. Definition of molality

Molality (m) is defined as:

m = moles of solute ÷ kilograms of solvent

Key points:

• Uses mass of solvent only, not the total solution
• Unit: mol/kg
• Does not change with temperature

Because mass remains the same even when temperature changes, molality is stable and reliable in temperature-dependent experiments.

2. Definition of molarity

Molarity (M) is defined as:

M = moles of solute ÷ litres of solution

Key points:

• Uses volume of the entire solution
• Unit: mol/L
• Changes with temperature

Volume expands when heated and contracts when cooled, so molarity varies with temperature.

3. Main differences between molality and molarity

Here are the key distinctions:

1. Basis of measurement

• Molality uses mass of solvent (kg)
• Molarity uses volume of solution (L)

1. Effect of temperature

• Molality is temperature-independent
• Molarity is temperature-dependent

This happens because volume changes with temperature, but mass does not.

1. Applications

• Molality is used in colligative property calculations such as boiling point elevation and freezing point depression.
• Molarity is used in titrations, laboratory solutions, and stoichiometric calculations.

1. Accuracy in physical chemistry

Molality is considered more accurate in experiments involving heating or cooling because its value remains constant.

4. Why molality is unaffected by temperature

Molality uses kilograms of solvent, and mass remains constant while temperature changes.
For example:

• If water at 20°C is heated to 80°C, its volume increases, but its mass stays the same.

As a result, molarity decreases with temperature rise, but molality stays unchanged.

This property makes molality ideal for studying colligative properties such as:

• Boiling point rise
• Freezing point depression
• Vapor pressure lowering
• Osmotic pressure

These depend only on the number of solute particles, not the solution’s volume.

5. Why molarity changes with temperature

Molarity uses litres of solution, and volume changes when heated or cooled.

• Heating increases volume → molarity decreases
• Cooling decreases volume → molarity increases

This means molarity is reliable only when temperature remains constant.

6. Practical examples

Example 1: Heating a solution

Suppose a salt solution is 1 M at 25°C.
If its temperature rises to 60°C:

• Volume expands
• Moles stay the same
• New molarity becomes less than 1 M

But if the same solution is described using molality, its value remains unchanged.

Example 2: Colligative property calculation

When calculating freezing point depression, molality must be used because the formula depends on the number of particles per kilogram of solvent.

Using molarity would give incorrect results due to temperature effects.

7. Summary of when to use each unit

Use molality (m) when:

• Temperature changes occur
• Studying colligative properties
• Working in physical chemistry
• High accuracy is needed

Use molarity (M) when:

• Preparing solutions in lab
• Performing titrations
• Doing general stoichiometry
• Temperature is stable

8. Relation between molality and molarity

Molality and molarity can be related mathematically, but the connection depends on:

• Density of the solution
• Amount of solute
• Temperature

Because these values vary, molality and molarity generally do not equal each other.

Conclusion

Molality and molarity are both units of concentration but differ in how they measure it. Molality uses moles of solute per kilogram of solvent and remains constant with temperature changes. Molarity uses moles of solute per litre of solution and varies with temperature. Molality is preferred for physical chemistry and colligative property studies, while molarity is ideal for general laboratory use and solution preparation. Understanding the difference ensures accurate calculations and correct application in various chemical processes.