Molarity and Formality, Formulas, Difference, Temperature Dependent, Examples for Class 11 Chemistry

ANAND CLASSES Study Material and Notes explore Molarity and Formality are essential concentration terms in Class 11 Chemistry. Molarity measures moles of solute per liter of solution, while Formality applies to ionic compounds. Both depend on temperature. Learn their formulas, key differences, and real-life examples for better understanding.

🔷 1. What is Molarity (M)?

Molarity (M) is a measure of the concentration of a solution. It tells us how many moles of solute are dissolved in one liter (1 L) of solution.

📌 Formula

$$M = \frac{\text{Number of moles of solute}}{\text{Volume of solution in litres}}$$

✅ Why is Molarity important?
Molarity helps in:

• Determining the exact amount of solute needed for chemical reactions.
• Calculating reactant and product amounts in chemical equations.
• Preparing standard solutions for titrations in laboratories.

🔷 2. How do we calculate Molarity?

The formula for molarity is:

$$M = \frac{n}{V}$$

where:

• n = number of moles of solute
• V = volume of solution in liters

🔹 Since we usually measure substances in grams, we convert mass into moles using the molar mass: $$n = \frac{\text{Mass of solute}}{\text{Molar mass of solute}}$$

📌 Thus, molarity can also be expressed as:

$$M = \frac{\text{Mass of solute}}{\text{Molar mass of solute} \times \text{Volume of solution in litres}}$$

🔷 3. What is the significance of Molarity in Chemistry?

✅ Titration experiments – Molarity helps determine the concentration of unknown solutions.
✅ Industrial applications – Used in the pharmaceutical and chemical industries for solution preparation.
✅ Stoichiometry calculations – Molarity helps in balancing chemical equations and determining reaction yields.

🔷 4. How do we apply the Molarity formula in real-life problems?

Example 1: Calculating Molarity

🔹 Suppose you dissolve 10 g of NaOH (sodium hydroxide) in 500 mL of water. What is the molarity of the solution?

✅ Step 1: Find the molar mass of NaOH: $$\text{NaOH} = 23 + 16 + 1 = 40 \quad \text{g/mol}$$

✅ Step 2: Calculate the number of moles: $$n = \frac{10}{40} = 0.25 \quad \text{moles}$$

✅ Step 3: Convert volume to litres: $$500 \text{ mL} = 0.5 \text{ L}$$

✅ Step 4: Use the molarity formula: $$M = \frac{0.25}{0.5} = 0.5 \quad \text{mol/L}$$

🔹 Final Answer: The molarity of the solution is 0.5 M.

🔷 5. Why does Molarity change with Temperature?

✅ Key Concept:
Molarity depends on volume, and volume changes with temperature.

• At higher temperatures, liquids expand, increasing the volume, which lowers the molarity.
• At lower temperatures, liquids contract, decreasing the volume, which increases the molarity.

🔹 Example:
A 1 M HCl solution at 25°C might become 0.98 M at 40°C due to expansion of volume.

📌 Conclusion: Molarity is temperature-dependent and should be used carefully in high-precision experiments.

🔷 6. What is Formality (F)? How is it different from Molarity?

🔹 Molarity is not useful for ionic compounds like NaCl because molecules do not exist separately in such cases. Instead, these compounds dissociate into ions.

🔹 Formality (F) is used for solutions of ionic compounds, where no distinct molecule exists. $$F = \frac{\text{Mass of solute}}{\text{Formula mass of solute} \times \text{Volume of solution in litres}}$$

✅ Example:

• NaCl (sodium chloride) dissociates into Na⁺ and Cl^– ions.
• Since there is no single NaCl molecule in solution, molarity loses its meaning, and formality is used instead.

📌 Key Difference:

🔹 Final Note:
For non-electrolytes (like sugar, glucose), molarity and formality have the same numerical value. But for electrolytes, molarity changes due to dissociation.

🔷 7. What are the major disadvantages of Molarity and Formality?

✅ Both change with temperature because volume expands/contracts.
✅ Molarity loses meaning for ionic compounds – this is why we use formality.
✅ Not suitable for precise industrial calculations, where volume changes can lead to errors.

🔷 8. What are some common misconceptions about Molarity and Formality?

❌ Misconception 1: “Molarity and Normality are the same.”
✔️ Reality: Normality depends on the equivalents of solute, while molarity depends only on moles.

❌ Misconception 2: “Molarity of NaCl solution is always the same.”
✔️ Reality: If NaCl dissociates into Na⁺ and Cl^–, the molarity of individual ions will be different.

❌ Misconception 3: “Formality and molarity are interchangeable.”
✔️ Reality: Molarity applies to molecular solutes, while Formality is for ionic compounds.

🔷 9. How can we use Molarity and Formality in competitive exams (JEE, NEET, CBSE)?

✅ MCQs & Numericals: Many questions in JEE/NEET require solving for molarity.
✅ Conceptual Questions: Understanding dissociation of ionic compounds is key for physical chemistry problems.
✅ Experimental Questions: CBSE board exams often ask about titration calculations using molarity.

🔷 10. Quick Revision Points 📌

✔️ Molarity (M) = Moles of solute per litre of solution.
✔️ Formality (F) = Formula mass of solute per litre of solution (used for ionic compounds).
✔️ Both change with temperature – volume expansion affects concentration.
✔️ Formality is useful when molecules do not exist independently (e.g., NaCl).
✔️ For non-electrolytes, molarity and formality have the same numerical value.

🌟 Final Thought

Molarity and Formality are crucial for understanding solution chemistry, with applications in medicine, engineering, and research. Mastering these concepts will give you a strong foundation for exams like JEE, NEET, and CBSE Board Chemistry! 🎯🔥

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