Short Answer
Molality (m) is a unit of concentration used in chemistry to show how many moles of solute are dissolved in 1 kilogram of solvent. It is expressed as:
molality (m) = moles of solute ÷ kilograms of solvent.
Molality is especially useful in experiments involving temperature changes because it depends only on the mass of the solvent, not the volume. Since mass does not change with temperature, molality remains constant even when the solution expands or contracts.
Detailed Explanation
Molality
Molality (m) is an important concentration unit that tells us how much solute is present in a fixed mass of solvent. Unlike molarity, which uses litres of solution, molality uses kilograms of solvent. This makes molality a more stable and reliable unit when temperature changes, because mass does not expand or contract.
Molality is mostly used in physical chemistry, especially in calculations related to boiling point elevation, freezing point depression, and colligative properties. These properties depend only on the number of particles in a solution, not on the volume. Therefore, molality is often preferred over molarity in these situations.
Definition of molality
Molality is defined as:
“The number of moles of solute dissolved in 1 kilogram of solvent.”
The formula is:
m = moles of solute ÷ kilograms of solvent
The unit of molality is mol/kg, but it is usually written simply as m (lowercase letter).
Example:
If 1 mole of sugar is dissolved in 1 kg of water, the solution is 1 m (1 molal).
How molality differs from molarity
Molality and molarity are both concentration units, but they differ in important ways:
| Property | Molarity (M) | Molality (m) |
| Based on | Litres of solution | Kilograms of solvent |
| Changes with temperature | Yes | No |
| Used in | General laboratory solutions | Colligative property calculations |
Molality does not change with temperature because mass remains constant. Molarity does change because volume can expand or contract.
How molality is calculated
To calculate molality, follow these steps:
Step 1: Find moles of solute
Convert the given mass of solute into moles using:
moles = mass ÷ molar mass
Example:
To find moles of NaCl,
Molar mass = 58.5 g/mol
If we have 29.25 g NaCl:
moles = 29.25 ÷ 58.5 = 0.5 mol
Step 2: Convert solvent mass to kilograms
Molality uses kilograms, not grams.
Example:
500 g water = 0.5 kg
Step 3: Apply molality formula
m = moles ÷ kilograms of solvent
Example:
0.5 mol ÷ 0.5 kg = 1 m
This means the solution is 1 molal.
Why molality does not depend on temperature
Molality is based on mass, and mass does not change when temperature changes.
Temperature affects volume, not mass.
This makes molality valuable in studies involving heat or temperature change, such as:
- Boiling point elevation
- Freezing point depression
- Vapor pressure lowering
- Osmotic pressure
These are called colligative properties, and they depend only on the number of solute particles.
Where molality is used
Molality is mainly used in:
- Colligative properties calculations
Molality directly affects boiling point rise and freezing point drop.
Example: Salt lowers the freezing point of water depending on its molality.
- Temperature-based experiments
Because molality does not change with temperature, it works reliably in experiments where heating or cooling occurs.
- Chemical engineering
Used when studying solvents in high-temperature reactions.
- Solutions at extreme conditions
High-pressure or high-temperature solutions are better described by molality.
- Pure solvent–solute interactions
Molality helps measure how solute particles affect solvent behaviour.
Real-world examples of molality
Example 1: Salt used on ice-covered roads
More salt (higher molality) causes a greater drop in freezing point, melting ice faster.
Example 2: Antifreeze in car radiators
Ethylene glycol mixed in water lowers its freezing point; this depends on molality.
Example 3: Cooking and food chemistry
Molality affects boiling behaviour in sugar syrups and salty water.
Example 4: Ocean water salinity
Chemists use molality to understand salt concentration in seawater.
Comparison summary
- Molarity (M) uses volume → changes with temperature
- Molality (m) uses mass → constant with temperature
Thus, molality is more reliable for physical chemistry calculations.
Conclusion
Molality (m) is a concentration unit that expresses the number of moles of solute per kilogram of solvent. It is especially important because it does not change with temperature, making it ideal for colligative property calculations such as boiling point elevation and freezing point depression. By using mass instead of volume, molality provides a stable and accurate measure of concentration in both laboratory and industrial applications.