Molarity vs Molality: Concept, Difference, and Practice Problems
Molarity and Molality are essential in chemistry and help students understand various practical and theoretical applications related to this topic. These concentration units play a crucial role in chemical solutions and are basic building blocks for a deeper grasp of quantitative chemistry.
What is Molarity and Molality in Chemistry?
Molarity (M) is the number of moles of solute present in one litre (1 L) of solution. Molality (m) is the number of moles of solute present in one kilogram (1 kg) of solvent. Both are popular units for expressing solution concentration, making them a part of your chemistry syllabus, including solution concentration units, mole concept, and stoichiometry.
Definitions and Formulas: Molarity & Molality
| Term | Definition | Formula | Unit |
|---|---|---|---|
| Molarity (M) | Moles of solute in 1 litre of solution | M = moles of solute / litres of solution | mol/L or M |
| Molality (m) | Moles of solute in 1 kilogram of solvent | m = moles of solute / kg of solvent | mol/kg or m |
To calculate moles of solute, use:
Moles = mass of solute (g) / molar mass (g/mol)
Key Differences Between Molarity and Molality
| Property | Molarity (M) | Molality (m) |
|---|---|---|
| Basis | Volume of solution (L) | Mass of solvent (kg) |
| Symbol | M (uppercase) | m (lowercase) |
| Unit | mol/L or M | mol/kg or m |
| Temperature effect | Affected (volume changes with temperature) | Not affected (mass remains constant) |
| Usage | Routine lab solutions | Colligative properties, precision work |
Numerical Example: How to Calculate Molarity
Let's calculate the molarity of a solution containing 18 g of glucose (C6H12O6) dissolved in enough water to make 0.5 L of solution.
1. Calculate molar mass of glucose: 180 g/mol2. Moles of glucose = 18 / 180 = 0.1 mol
3. Molarity (M) = 0.1 mol / 0.5 L = 0.2 M
Numerical Example: How to Calculate Molality
Suppose 18 g of glucose is dissolved in 200 g of water. What is the molality?
1. Moles of glucose = 18 / 180 = 0.1 mol2. Mass of solvent = 200 g = 0.2 kg
3. Molality (m) = 0.1 mol / 0.2 kg = 0.5 m
Temperature and Molarity vs. Molality
Molarity depends on solution volume, which changes with temperature. If temperature increases, solution expands and molarity decreases. Molality relies only on mass, so it stays constant with temperature or pressure changes.
When to Use Molarity or Molality
- Use molarity for common laboratory solutions and reactions where the volume is easily measured.
- Choose molality for physical chemistry topics—like boiling point elevation, freezing point depression, or when temperature may change.
- Use molality for high-precision or colligative property calculations.
Try This Yourself
- What is the molarity of 5 g NaCl (molar mass 58.5 g/mol) dissolved in 250 mL solution?
- How many grams of urea (molar mass 60 g/mol) to make 2 m solution in 250 g water?
Volume = 0.25 L;
M = 0.0855 / 0.25 = 0.342 M
Solvent = 0.25 kg;
Moles = 2 x 0.25 = 0.5;
Mass = 0.5 x 60 = 30 g
Summary Table: Molarity vs. Molality at a Glance
| Feature | Molarity (M) | Molality (m) |
|---|---|---|
| Formula | M = moles/volume (L) | m = moles/mass (kg) |
| Unit | mol/L | mol/kg |
| Volume or mass based? | Volume (solution) | Mass (solvent) |
| Temperature effect | Changes with temperature | Not affected by temperature |
Relation with Other Chemistry Concepts
Molarity and molality connect strongly with the mole concept, solution concentration properties, colligative properties, and molar mass formula. They are the foundation for quantitative chemical calculations and physical chemistry experiments.
Lab or Experimental Tips
Always use a volumetric flask for precise molarity solutions and an analytical balance for molality. Remember: temperature changes volume, but not mass! Vedantu educators recommend always stating the unit (M or m) in answers to avoid confusion.
Final Wrap-Up
We explored molarity and molality—their definitions, formulas, key differences, and applications in practical chemistry. Both are vital for making and understanding chemical solutions. For more help, check out live classes and detailed notes on Vedantu and connect these concepts with solution types as you keep learning!
FAQs on Molarity and Molality Explained with Examples
1. What is the difference between molarity and molality?
Molarity is defined as moles of solute per litre of solution, while molality is moles of solute per kilogram of solvent. Key differences:
- Molarity (M): moles of solute / litres of solution
- Molality (m): moles of solute / kilograms of solvent
- Molarity changes with temperature; molality does not.
- Units: M (mol/L), m (mol/kg)
2. How do you calculate molarity?
To calculate molarity (M):
- Find the moles of solute.
- Measure the total volume of solution in litres.
- Use the formula: M = moles of solute / volume of solution (L)
Example: If you dissolve 0.5 mol NaCl in 1 L water, M = 0.5 / 1 = 0.5 M
3. How do you calculate molality?
Molality (m) is calculated as:
- Find moles of solute.
- Measure mass of solvent in kilograms.
- Apply the formula: m = moles of solute / mass of solvent (kg)
Example: 0.1 mol of glucose in 0.5 kg water: m = 0.1 / 0.5 = 0.2 m
4. Is molarity affected by temperature?
Yes, molarity is temperature-dependent because the volume of a solution changes with temperature, altering concentration. In contrast, molality is not affected by temperature as it depends on mass, which remains constant.
5. Which is more accurate: molarity or molality?
Molality is more accurate for experiments involving temperature change because it is based on the mass of solvent. Molarity may vary due to volume expansion or contraction.
- Use molality in high-precision and colligative property studies.
- Use molarity for most standard lab solutions at constant temperature.
6. What is a 1 molar solution?
A 1 molar (1 M) solution contains exactly 1 mole of solute dissolved in enough solvent to make 1 litre of total solution.
7. Can molarity and molality ever have the same value?
Molarity and molality can be nearly equal in very dilute aqueous solutions where the density of the solution is close to 1 g/mL and the solute mass is negligible compared to the solvent.
8. How do you convert molarity to molality?
To convert molarity (M) to molality (m):
- Find the solution's density (g/mL or kg/L).
- Use: molality = [molarity × 1000] / [density (g/L) × 1000 – molarity × molar mass (g)]
This helps relate concentration units in different conditions.
9. Why is molality preferred over molarity in colligative property calculations?
Molality is preferred for colligative properties because it depends only on the mass of the solvent, not the solution's volume. This ensures results are independent of temperature and pressure changes.
10. List the main formulas for molarity and molality.
Key formulas:
- Molarity (M) = moles of solute / litres of solution
- Molality (m) = moles of solute / kilograms of solvent
- For conversions, use solution density and molar mass as needed.
11. What are some examples of molarity and molality in daily life or laboratory practice?
Examples:
- Preparing saline solutions: Calculated using molarity for hospital IV fluids.
- Colligative property experiments: Molality used to determine depression in freezing point.
- Titrations: Molarity helps measure reactant concentrations accurately.
12. What is the significance of solution density in molarity and molality calculations?
Solution density is important when converting between molarity and molality. This is because:
- Molarity uses solution volume (affected by density).
- Molality uses solvent mass (does not depend on density).
Knowing density ensures correct conversion and accurate concentration values.
