Difference Between Molarity and Molality With Solved Examples
The Molarity and Molality of solutions are central concepts in JEE Main Chemistry. They help you express how much solute is present in a solution, enabling precise chemical calculations. Both concentration units are essential for mastering quantitative aspects of stoichiometry and for correctly tackling a variety of solution-based problems in JEE and practical chemistry. Understanding their formulas, differences, and applications is key to scoring in solution chemistry.
Definitions and Notation: Molarity and Molality
Molarity (M) is defined as the number of moles of solute per litre of solution. The formula is M = n/V, where n is moles of solute and V is the total solution volume in litres (L).
Molality (m) is the number of moles of solute per kilogram of solvent. Its formula is m = n/w, where w is the mass of solvent in kilograms (kg).
Key Formulas for JEE Solution Chemistry
- Molarity (M) = Moles of solute / Volume of solution (L)
- Molality (m) = Moles of solute / Mass of solvent (kg)
- 1 Litre (L) = 1000 mL; 1 kg = 1000 g
- Use molecular mass to convert grams to moles: n = mass (g) / molar mass (g/mol)
Always check units before using any formula. For dilute aqueous solutions, 1 litre of water weighs roughly 1 kilogram, but never assume this for concentrated or non-aqueous solutions. JEE numericals often test such subtle traps.
Table: Molarity vs Molality – JEE Comparison
| Property | Molarity (M) | Molality (m) |
|---|---|---|
| Definition | Moles of solute / Litre of solution | Moles of solute / kg of solvent |
| SI Unit | mol L–1 or M | mol kg–1 or m |
| Symbol | M | m |
| Depends on | Volume of solution | Mass of solvent |
| Temperature effect | Affected (volume expands/contracts) | Not affected (mass stays same) |
| Use Case | Routine lab work, titrations | Colligative properties, theoretical studies |
The primary difference between molarity and molality is their dependence: molarity uses the total solution volume, so it's affected by temperature; molality depends only on solvent mass and is independent of temperature. This makes molality crucial for precise physical chemistry calculations.
Solved Numericals: Molarity and Molality Steps
Example 1 (Molarity Calculation): 18 g of glucose (C6H12O6, molar mass = 180 g/mol) is dissolved to make 0.5 L of solution. Find molarity.
- Moles of glucose = 18 ÷ 180 = 0.1 mol
- Volume = 0.5 L
- M = 0.1 ÷ 0.5 = 0.2 M
Final answer: 0.2 M solution.
Example 2 (Molality Calculation): 18 g glucose dissolved in 200 g water. Find molality.
- Moles of glucose = 18 ÷ 180 = 0.1 mol
- Mass solvent = 200 g = 0.2 kg
- m = 0.1 ÷ 0.2 = 0.5 m
Molality of the solution = 0.5 m.
Temperature Impact and Conversion Formula
Always remember: Molarity decreases as temperature increases because solution expands, changing its volume. Molality remains constant since solvent mass doesn't change with temperature. For conversion between these units, use the formula:
- m = M × (density of solution in g/mL) × 1000 / [1000 × (1 – M × molar mass / density)]
- Conversion questions often require knowing the density of the solution.
Where to Use: JEE Main Applications
For standard laboratory reactions, molarity is the routine unit—measured easily by using a volumetric flask for solution. For questions on colligative properties and where temperature changes, always use molality for accuracy. JEE Main twists often test this distinction in exam scenarios.
Error Traps and Key Tips for JEE
- Don't confuse solvent mass with solution mass when using molality.
- Always state units ("M" or "m") in answers for clarity and marks.
- Memorise that molarity depends on volume (so, temperature and pressure matter), while molality is mass-based and remains unchanged under such variations.
- Carefully read JEE questions for which unit is required; they may ask to convert between molarity and molality.
Practice Problems: Try These Next
- Find the molality if 10 g NaCl (molar mass 58.5 g/mol) is dissolved in 250 g water.
- A solution contains 9.8 g H2SO4 in 500 mL. Calculate its molarity.
- Convert 0.5 M KNO3 solution (density 1.1 g/mL) to molality.
For more extensive JEE-style practice, visit the Solutions Mock Test Series and Concentration Terms resource.
Molarity and Molality in Broader JEE Chemistry
- Connect these units with the Mole Concept to solve multi-step stoichiometry.
- Apply in colligative property numericals (boiling point elevation, freezing point depression).
- Brush up foundational concepts at Some Basic Concepts in Chemistry for related definition questions.
- Use with Stoichiometry problems to analyse limiting reagents or yields.
Summary Table: Quick JEE Revision – Molarity vs Molality
| Feature | Molarity | Molality |
|---|---|---|
| Formula | n / V (L) | n / w (kg) |
| Depends on | Total volume of solution | Mass of solvent only |
| Effect of temp | Changes | Unaffected |
| JEE Use | Lab solutions, titrations | Colligative properties |
Mastering Molarity and Molality is vital for success in JEE Main solution chemistry, titrations, and all colligative property problems. Always validate your units and recall their key dependence differences, especially under temperature change conditions. Explore more at Vedantu’s curated Solutions article and advance your practice for top JEE performance.
FAQs on Molarity and Molality Explained for JEE and Class 11 Chemistry
1. What is molarity in chemistry?
Molarity in chemistry is the measure of concentration that expresses the number of moles of solute present in one litre of solution. It is widely used in exam numericals and laboratory calculations.
- Formula: Molarity (M) = Moles of solute / Volume of solution (in litres)
- Unit: mol/L or M
- Used for easy calculation of reactant and product quantities
2. What is the difference between molarity and molality?
Molarity and molality are both concentration units but differ in their reference and temperature dependency.
- Molarity: Moles of solute per litre of solution; temperature dependent
- Molality: Moles of solute per kilogram of solvent; temperature independent
- Molality is preferred in experiments where temperature changes
3. What is the formula for calculating molality?
Molality (m) is calculated using the moles of solute dissolved in a specific mass of solvent.
- Formula: Molality (m) = Moles of solute / Mass of solvent (kg)
- Unit: mol/kg
- Useful in colligative property numericals
4. How do you convert molarity to molality?
To convert molarity to molality, you need the density of the solution and the molar mass values.
- Find moles of solute per litre of solution (molarity).
- Calculate mass of solvent using density data.
- Apply: Molality = (Molarity × 1000) / (Density × 1000 – Molarity × Molar mass of solute).
5. Is molarity affected by temperature changes?
Yes, molarity changes with temperature because the volume of a solution expands or contracts as temperature varies.
- Molarity = Moles of solute / Volume of solution (in litres)
- Since volume is temperature-dependent, so is molarity
- Molality does not depend on temperature
6. What are the quick tips for solving molarity and molality problems?
Use these tips for fast and accurate molarity and molality calculations:
- Read questions carefully to spot solvent vs. solution distinction
- Always convert units to litres (for molarity) and kilograms (for molality)
- Memorize standard formulas for both terms
- Double-check density usage in conversion questions
- Practice multiple solved examples before exams
7. What is 1 molarity equal to?
1 molarity (1 M) means one mole of solute is dissolved in one litre of solution.
- Unit: mol/L
- For example, 1 M NaCl: 1 mole of NaCl in 1 L solution
- Most laboratory solutions are prepared using their molarity
8. Why do some exam questions specify molality instead of molarity?
Exam questions specify molality when temperature independence is important, such as in colligative properties.
- Molality is unaffected by temperature changes since it depends on mass, not volume
- Ensures calculations remain accurate despite temperature fluctuations
9. Can you use molality in every chemical calculation?
Molality is very useful in certain calculations, especially when working with temperature effects, but not always suitable for all solution-related problems.
- Best used for: Colligative properties, boiling point elevation, freezing point depression
- Not always ideal for: Titration or where the solution's exact volume is necessary
10. What mistake do students often make when identifying solvent vs. solution?
Students often confuse solvent (used in molality) with solution (used in molarity), leading to unit errors.
- Molarity uses total solution volume
- Molality uses mass of only the solvent
- Always check question wording for clarity before calculation
