Laws of Chemical Combinations Class 11 CBSE Notes with MCQs and Answers
 |
| Laws of Chemical Combinations – Complete CBSE Class 11 Chemistry Notes with MCQs, Case Studies, and Exam Questions. |
- Dr.Sanjaykumar Pawar
Laws of Chemical Combinations (NEET Notes)
📖 Introduction
The Laws of Chemical Combinations explain how elements combine to form compounds. These laws are the foundation of chemistry and are very important for the NEET examination.
1. Law of Conservation of Mass
Statement
During any physical change or chemical reaction, the total mass remains constant.
Matter can neither be created nor destroyed.
Formula
Mass of Reactants = Mass of Products
Example
Hydrogen + Oxygen → Water
- 2 g Hydrogen + 16 g Oxygen = 18 g Water
Total mass before reaction = 18 g
Total mass after reaction = 18 g
Hence, mass is conserved.
Key Points
- Proposed by Antoine Lavoisier (1789).
- Applicable to both physical and chemical changes.
- Atoms only rearrange; they are not created or destroyed.
NEET Shortcut
✅ Mass before reaction = Mass after reaction
2. Law of Definite Proportions (Law of Constant Composition)
Statement
A pure chemical compound always contains the same elements in the same fixed proportion by mass, irrespective of its source or method of preparation.
Also called Law of Constant Composition.
Example
Water (H₂O)
No matter where water comes from—
- River
- Sea
- Rain
- Laboratory
It always contains
Hydrogen : Oxygen
2 : 16 = 1 : 8 (by mass)
Another Example
Carbon dioxide (CO₂)
Carbon = 12 g
Oxygen = 32 g
Ratio = 12 : 32 = 3 : 8
This ratio is always fixed.
Key Points
- Proposed by Joseph Proust (1799).
- Only pure compounds obey this law.
- Mixtures do not follow this law.
NEET Shortcut
✅ Same compound → Same mass ratio
3. Law of Multiple Proportions
Statement
When two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in the ratio of small whole numbers.
Example 1: Water and Hydrogen Peroxide
Water (H₂O)
Hydrogen = 2 g
Oxygen = 16 g
Hydrogen Peroxide (H₂O₂)
Hydrogen = 2 g
Oxygen = 32 g
Hydrogen mass is fixed.
Oxygen masses are
16 : 32
= 1 : 2
(Simple whole number ratio)
Hence, law is verified.
Example 2: Carbon Monoxide and Carbon Dioxide
CO
Carbon = 12 g
Oxygen = 16 g
CO₂
Carbon = 12 g
Oxygen = 32 g
Carbon mass is fixed.
Oxygen masses
16 : 32
= 1 : 2
Again, a simple whole number ratio.
Key Points
- Proposed by John Dalton (1803).
- Applies only when more than one compound is formed from the same two elements.
NEET Shortcut
✅ Same two elements + Different compounds → Small whole number ratio
4. Gay-Lussac's Law of Gaseous Volumes
Statement
When gases react or are produced in a chemical reaction, their volumes bear a simple whole number ratio, provided the temperature (T) and pressure (P) remain constant.
Conditions
- Temperature must be constant.
- Pressure must be constant.
- Only gases are considered.
Example 1
Hydrogen + Chlorine → Hydrogen chloride
1 volume H₂ + 1 volume Cl₂ → 2 volumes HCl
Ratio
1 : 1 : 2
(Simple whole numbers)
Example 2
2 volumes H₂ + 1 volume O₂ → 2 volumes H₂O (steam)
Ratio
2 : 1 : 2
Again, simple whole numbers.
Key Points
- Proposed by Joseph Louis Gay-Lussac (1808).
- Later explained by Avogadro's Law.
NEET Shortcut
✅ Volumes of gases combine in simple whole-number ratios
5. Avogadro's Law
Statement
Equal volumes of all gases, at the same temperature and pressure, contain an equal number of molecules.
Meaning
If two gas containers have
- Same temperature
- Same pressure
- Same volume
then both contain the same number of molecules, regardless of the gas.
Example
1 L Hydrogen
1 L Oxygen
1 L Nitrogen
at the same temperature and pressure
All contain equal numbers of molecules.
Mathematical Form
Volume ∝ Number of moles
or
V ∝ n
where:
- V = Volume
- n = Number of moles
Important Results
At STP:
- 1 mole of any gas = 22.4 L (ideal gas)
Examples:
- 22.4 L O₂ = 1 mole
- 22.4 L N₂ = 1 mole
- 22.4 L H₂ = 1 mole
All contain 6.022 × 10²³ molecules (Avogadro's number).
Key Points
- Proposed by Amedeo Avogadro (1811).
- Distinguished between atoms and molecules.
- Helped explain Gay-Lussac's law.
NEET Shortcut
✅ Equal volume + Same T & P = Equal number of molecules
Quick Comparison Table
| Law | Scientist | Main Idea | NEET Keyword |
|---|---|---|---|
| Law of Conservation of Mass | Antoine Lavoisier | Mass remains constant | Reactants = Products |
| Law of Definite Proportions | Joseph Proust | Fixed mass ratio | Constant Composition |
| Law of Multiple Proportions | John Dalton | Small whole-number ratio | Different compounds |
| Gay-Lussac's Law | Joseph Louis Gay-Lussac | Gas volumes combine in simple ratios | Same T & P |
| Avogadro's Law | Amedeo Avogadro | Equal volumes contain equal molecules | V ∝ n |
NEET Important Differences
| Definite Proportions | Multiple Proportions |
|---|---|
| One compound | Two or more compounds |
| Same mass ratio | Different mass ratios |
| Fixed composition | Small whole-number ratio |
| Example: H₂O | Example: H₂O and H₂O₂ |
NEET Memory Tricks
- Lavoisier → Mass is conserved.
- Proust → Pure compound has fixed composition.
- Dalton → Different compounds give small whole-number ratios.
- Gay-Lussac → Gas volumes combine in simple ratios.
- Avogadro → Equal volumes contain equal numbers of molecules.
Frequently Asked NEET Facts
- Law of Conservation of Mass → Antoine Lavoisier
- Law of Definite Proportions → Joseph Proust
- Law of Multiple Proportions → John Dalton
- Law of Gaseous Volumes → Joseph Louis Gay-Lussac
- Avogadro's Law → Amedeo Avogadro
- Avogadro's Number = 6.022 × 10²³
- Molar Volume at STP (ideal gas) = 22.4 L mol⁻¹
One-Minute Revision
- Conservation of Mass: Total mass remains constant.
- Definite Proportions: A compound always has the same mass ratio of elements.
- Multiple Proportions: Different compounds of the same elements show small whole-number mass ratios.
- Gay-Lussac: Gases combine in simple volume ratios at the same temperature and pressure.
- Avogadro: Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.
CBSE Class 11 Chemistry – Laws of Chemical Combinations
Question Bank with Answers (CBSE + NEET Level)
A. Multiple Choice Questions (MCQs)
1. Who proposed the Law of Conservation of Mass?
A) Dalton B) Proust C) Lavoisier D) Avogadro
Answer: C) Lavoisier
2. According to the Law of Conservation of Mass,
A) Matter is destroyed. B) Mass of products is greater than reactants. C) Total mass remains constant. D) Atoms disappear.
Answer: C) Total mass remains constant.
3. Which law is also known as the Law of Constant Composition?
A) Multiple Proportions B) Conservation of Mass C) Definite Proportions D) Avogadro's Law
Answer: C) Definite Proportions
4. Water always contains hydrogen and oxygen in the mass ratio
A) 1 : 16
B) 1 : 2
C) 1 : 8
D) 2 : 8
Answer: C) 1 : 8
5. Which law is obeyed by pure compounds only?
A) Gay-Lussac's Law
B) Definite Proportions
C) Avogadro's Law
D) Conservation of Mass
Answer: B) Definite Proportions
6. The Law of Multiple Proportions was proposed by
A) Dalton
B) Avogadro
C) Gay-Lussac
D) Proust
Answer: A) Dalton
7. In CO and CO₂, the mass of oxygen combining with fixed carbon is
A) 1 : 2
B) 2 : 1
C) 3 : 8
D) 1 : 1
Answer: A) 1 : 2
8. Gay-Lussac's Law deals with
A) Mass
B) Atoms
C) Volumes of gases
D) Molecules
Answer: C) Volumes of gases
9. Equal volumes of gases at the same temperature and pressure contain
A) Equal masses
B) Equal atoms
C) Equal molecules
D) Equal densities
Answer: C) Equal molecules
10. Avogadro's Number is
A) 6.022 × 10²⁰
B) 6.022 × 10²¹
C) 6.022 × 10²²
D) 6.022 × 10²³
Answer: D) 6.022 × 10²³
11. One mole of an ideal gas at STP occupies
A) 11.2 L
B) 22.4 L
C) 44.8 L
D) 24 L
Answer: B) 22.4 L
12. Which law explains Gay-Lussac's Law?
A) Dalton's Law
B) Avogadro's Law
C) Boyle's Law
D) Charles' Law
Answer: B) Avogadro's Law
B. Very Short Answer Questions (1 Mark)
1. State the Law of Conservation of Mass.
Answer: Mass can neither be created nor destroyed during a chemical reaction.
2. Who proposed the Law of Definite Proportions?
Answer: Joseph Proust.
3. What is the mass ratio of hydrogen and oxygen in water?
Answer: 1 : 8.
4. Define Avogadro's Law.
Answer: Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.
5. Write Avogadro's Number.
Answer: 6.022 × 10²³.
6. Write the mathematical form of Avogadro's Law.
Answer: V ∝ n.
7. Which law is applicable only to gases?
Answer: Gay-Lussac's Law.
8. Name the scientist who proposed the Law of Multiple Proportions.
Answer: John Dalton.
9. Write the molar volume of an ideal gas at STP.
Answer: 22.4 L mol⁻¹.
10. Which law states that compounds have fixed composition?
Answer: Law of Definite Proportions.
C. Short Answer Questions (2–3 Marks)
1. State the Law of Conservation of Mass with an example.
Answer: The total mass of reactants is equal to the total mass of products.
Example: 2 g Hydrogen + 16 g Oxygen → 18 g Water
Mass before reaction = Mass after reaction = 18 g.
2. Explain the Law of Definite Proportions.
Answer: A pure compound always contains the same elements in the same fixed proportion by mass irrespective of its source.
Example: Water always contains hydrogen and oxygen in the mass ratio 1 : 8.
3. Explain the Law of Multiple Proportions using CO and CO₂.
Answer: 12 g carbon combines with
16 g oxygen in CO
32 g oxygen in CO₂
Ratio = 16 : 32 = 1 : 2
Hence, the law is verified.
4. State Gay-Lussac's Law.
Answer: When gases react at constant temperature and pressure, their volumes bear a simple whole-number ratio.
Example: 2 volumes H₂ + 1 volume O₂ → 2 volumes H₂O.
5. Write three important results of Avogadro's Law.
Answer:
- Equal volumes contain equal molecules.
- V ∝ n.
- One mole of gas occupies 22.4 L at STP.
D. Long Answer Questions (5 Marks)
1. Explain the Law of Conservation of Mass with suitable examples.
Answer:
- Proposed by Antoine Lavoisier.
- Matter cannot be created or destroyed.
- Total mass remains constant.
- Applicable to both physical and chemical changes.
Example: 2 g H₂ + 16 g O₂ → 18 g H₂O.
Importance:
- Basis of balancing chemical equations.
- Supports atomic theory.
2. Explain the Law of Definite Proportions and Multiple Proportions with examples.
Answer:
Law of Definite Proportions A compound always contains the same elements in fixed mass ratio.
Example: Water = H : O = 1 : 8.
Law of Multiple Proportions When two elements form more than one compound, masses combine in simple whole-number ratios.
Example:
CO = 12 g C +16 g O
CO₂ =12 g C +32 g O
Ratio =1 :2.
Difference:
- Definite Proportions → One compound.
- Multiple Proportions → More than one compound.
3. Explain Avogadro's Law and Gay-Lussac's Law.
Answer: Gay-Lussac's Law Gas volumes combine in simple whole-number ratios at constant T and P.
Example: 1 volume H₂ +1 volume Cl₂ →2 volumes HCl.
Avogadro's Law Equal volumes of gases at same T and P contain equal molecules.
Formula: V ∝ n.
Applications:
- Molar volume.
- Molecular formula determination.
- Explains Gay-Lussac's Law.
E. Assertion and Reason Questions
1.
Assertion (A): Mass is conserved in a chemical reaction.
Reason (R): Atoms are neither created nor destroyed during a chemical reaction.
Answer: Both A and R are true, and R is the correct explanation of A.
2.
Assertion: Water from every source has the same composition.
Reason: Water is a pure compound.
Answer: Both A and R are true, and R is the correct explanation.
3.
Assertion: CO and CO₂ obey the Law of Multiple Proportions.
Reason: Oxygen combines with fixed carbon in the ratio 1 : 2.
Answer: Both A and R are true, and R is the correct explanation.
4.
Assertion: Equal volumes of gases contain equal masses.
Reason: Equal volumes contain equal molecules.
Answer: Assertion is false, but Reason is true.
5.
Assertion: Gay-Lussac's Law applies only to gases.
Reason: It deals with combining volumes of gases.
Answer: Both A and R are true, and R is the correct explanation.
F. Fill in the Blanks
-
The Law of Conservation of Mass was proposed by Antoine Lavoisier.
-
The Law of Constant Composition is also called the Law of Definite Proportions.
-
Equal volumes of gases contain equal numbers of molecules.
-
One mole of gas occupies 22.4 L at STP.
-
Avogadro's Number is 6.022 × 10²³.
-
Water contains hydrogen and oxygen in the ratio 1 : 8 by mass.
-
Gay-Lussac's Law applies only to gases.
-
Volume is directly proportional to the number of moles.
-
The Law of Multiple Proportions was proposed by John Dalton.
-
Definite Proportions apply only to pure compounds.
G. Match the Columns
| Column A | Column B |
|---|---|
| A. Lavoisier | 1. Conservation of Mass |
| B. Proust | 2. Definite Proportions |
| C. Dalton | 3. Multiple Proportions |
| D. Gay-Lussac | 4. Gaseous Volumes |
| E. Avogadro | 5. Equal Volumes |
Answers
A–1
B–2
C–3
D–4
E–5
H. Statement-Based Questions
1. Identify the correct statement.
A. Mass increases after reaction.
B. Mass decreases after reaction.
C. Mass remains constant.
D. Mass disappears.
Answer: C
2. Which statement is incorrect?
A. Pure compounds have fixed composition.
B. Mixtures obey definite proportions.
C. Water has fixed composition.
D. CO₂ has fixed composition.
Answer: B
3. Which statement is correct?
A. Equal gas volumes contain equal molecules.
B. Equal gas volumes contain equal masses.
C. Equal gas volumes contain equal atoms.
D. None.
Answer: A
I. Case Study Questions
Case Study 1
Rahul mixed hydrogen and oxygen gases in a closed container. After the reaction, he found that 2 g hydrogen combined with 16 g oxygen to produce 18 g water.
Questions
1. Which law is verified?
Answer: Law of Conservation of Mass.
2. Who proposed this law?
Answer: Antoine Lavoisier.
3. What is the total mass before reaction?
Answer: 18 g.
4. What is the total mass after reaction?
Answer: 18 g.
5. Why is mass conserved?
Answer: Because atoms are only rearranged during the reaction.
Case Study 2
A student compares CO and CO₂.
CO contains
12 g carbon
16 g oxygen
CO₂ contains
12 g carbon
32 g oxygen
Questions
1. Which law is illustrated?
Answer: Law of Multiple Proportions.
2. Who proposed it?
Answer: John Dalton.
3. Calculate the oxygen ratio.
Answer: 16 : 32 = 1 : 2.
4. Which element is kept constant?
Answer: Carbon.
5. What type of ratio is obtained?
Answer: A simple whole-number ratio.
J. Important CBSE Competency-Based Questions
1. Why do mixtures not obey the Law of Definite Proportions?
Answer: Because the composition of mixtures is variable and not fixed.
2. Why is Avogadro's Law important?
Answer: It relates the volume of a gas to the number of moles and helps determine molecular formulas.
3. Explain why CO and CO₂ do not violate the Law of Definite Proportions.
Answer: Each compound individually has a fixed composition. The law compares one compound at a time, while the Law of Multiple Proportions compares different compounds formed by the same elements.
4. Differentiate between the Law of Definite Proportions and the Law of Multiple Proportions.
| Law of Definite Proportions | Law of Multiple Proportions |
|---|---|
| Applies to one compound | Applies to two or more compounds |
| Fixed mass ratio | Small whole-number ratio |
| Example: H₂O | Example: CO and CO₂ |
Exam Tip (CBSE)
Focus on:
- Scientist associated with each law.
- Exact statements of the laws.
- Numerical examples involving mass ratios and gas volumes.
- Differences between the laws.
- Assertion–Reason and Case Study questions, as these are commonly asked in CBSE Class 11 examinations.
Comments
Post a Comment