Laws of Chemical Combination with Numerical Problems | JKBOSE Class 11 Chemistry

.ast-archive-entry-banner[data-post-type="post"][data-banner-background-type="custom"]{--wpr-bg-49bb913e-ada5-4f50-9021-29a746d68147: url('https://asteriskclasses.com/wp-content/uploads/2024/05/pexels-photo-4144101.jpeg');}

Laws of Chemical Combination (Numerical Problems)

Fundamental Laws Governing Chemical Reactions

These laws established the quantitative foundation of chemistry and led to Dalton’s Atomic Theory. They were developed through meticulous experiments by Lavoisier, Proust, and Dalton.

Historical Context: Developed between 1789-1803 during the Chemical Revolution

1. Law of Conservation of Mass (Lavoisier, 1789)

Statement: In a chemical reaction, matter is neither created nor destroyed. The total mass of reactants equals the total mass of products.

∑Mass_reactants = ∑Mass_products

Key Features:

Valid for closed systems (no mass exchange with surroundings)
Forms basis for stoichiometric calculations
Exception: Nuclear reactions (mass-energy conversion)

Example 1: Thermal Decomposition

When 24.8g mercury(II) oxide decomposes, it produces 23.0g mercury and 1.8g oxygen. Verify the law.

Solution:
2HgO → 2Hg + O₂
Mass reactants = 24.8g
Mass products = 23.0g + 1.8g = 24.8g
Law verified (24.8g = 24.8g)

Example 2: Precipitation Reaction

10.0g silver nitrate reacts with 5.0g sodium chloride to form 8.5g silver chloride and sodium nitrate. Calculate the mass of sodium nitrate produced.

Solution:
AgNO₃ + NaCl → AgCl + NaNO₃
Total mass reactants = 10.0g + 5.0g = 15.0g
Mass NaNO₃ = 15.0g – 8.5g = 6.5g

2. Law of Definite Proportions (Proust, 1799)

Statement: A pure chemical compound always contains the same elements combined together in the same fixed proportion by mass.

For H₂O: Mass ratio H:O = 1:8 (always)

Key Features:

Independent of source or method of preparation
Exception: Non-stoichiometric compounds (e.g., wüstite Fe_0.95O)
Led to concept of chemical purity

Example 1: Carbon Dioxide

Sample A of CO₂ contains 3g carbon and 8g oxygen. Sample B contains 6g carbon and 16g oxygen. Verify the law.

Solution:
Sample A ratio C:O = 3:8
Sample B ratio C:O = 6:16 = 3:8
Identical ratios confirm the law

Example 2: Copper Carbonate

Natural malachite (CuCO₃) and synthetic copper carbonate both show Cu:C:O mass ratio as 64:12:48. What does this prove?

Solution:
Simplified ratio = 16:3:12 (dividing by 4)
Confirms identical composition regardless of source

3. Law of Multiple Proportions (Dalton, 1803)

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 are in a ratio of small whole numbers.

CO (carbon monoxide) vs CO₂ (carbon dioxide):
Oxygen mass ratios for fixed carbon = 1:2

Key Features:

Direct evidence for atomic theory
Explains existence of different compounds from same elements
Foundation for chemical formulas

Example 1: Nitrogen Oxides

NO contains N:O = 14:16. N₂O contains N:O = 28:16. Show they obey the law.

Solution:
For fixed O mass (16g):
N masses are 14g (NO) and 28g (N₂O)
Ratio = 14:28 = 1:2 (simple whole number)

Example 2: Sulfur Fluorides

SF₆ has S:F = 32:114. SF₄ has S:F = 32:76. Verify the law.

Solution:
For fixed S mass (32g):
F masses are 114g and 76g
Ratio = 114:76 = 3:2 (simple whole numbers)

10 Solved Numerical Problems

Problem 1: Conservation of Mass

3.0g magnesium ribbon burns in oxygen to produce 5.0g magnesium oxide. Calculate the mass of oxygen consumed.

Step 1: Write reaction
2Mg + O₂ → 2MgO

Step 2: Apply mass conservation
Mass MgO = Mass Mg + Mass O₂
5.0g = 3.0g + x
x = 2.0g oxygen

Problem 2: Definite Proportions

Two samples of water were decomposed. Sample A gave 2g hydrogen and 16g oxygen. Sample B gave 5g hydrogen and 40g oxygen. Verify the law.

Solution:
Sample A ratio H:O = 2:16 = 1:8
Sample B ratio H:O = 5:40 = 1:8
Identical ratios confirm the law

Problem 3: Conservation in Neutralization

2.0g sodium hydroxide reacts with 3.65g hydrochloric acid to produce 5.85g sodium chloride and water. Calculate the mass of water formed.

Step 1: Write reaction
NaOH + HCl → NaCl + H₂O

Step 2: Apply mass conservation
Total mass reactants = 2.0g + 3.65g = 5.65g
Mass H₂O = 5.65g – 5.85g NaCl = 0.80g water

Problem 4: Hydrate Composition

Copper sulfate pentahydrate (CuSO₄·5H₂O) always contains 36.1% water by mass. A sample weighing 10.0g is dehydrated. Calculate the mass of anhydrous CuSO₄ obtained.

Solution:
Mass % water = 36.1% → Mass water = 10.0g × 0.361 = 3.61g
Mass CuSO₄ = 10.0g – 3.61g = 6.39g

Problem 5: Carbon Oxides Ratio

CO contains C:O = 12:16. CO₂ contains C:O = 12:32. Show they obey multiple proportions law.

Solution:
For fixed C mass (12g):
O masses = 16g (CO) and 32g (CO₂)
Ratio = 16:32 = 1:2 (simple whole number)

Problem 6: Combustion Analysis

1.6g methane (CH₄) burns in 6.4g oxygen to produce 4.4g CO₂ and water. Calculate the mass of water formed.

Step 1: Write reaction
CH₄ + 2O₂ → CO₂ + 2H₂O

Step 2: Apply mass conservation
Total mass reactants = 1.6g + 6.4g = 8.0g
Mass H₂O = 8.0g – 4.4g = 3.6g water

Problem 7: Mineral Composition

Malachite (Cu₂(OH)₂CO₃) from two sources shows identical Cu:O:H:C ratios. Sample A has 57.5g Cu, 20.0g O, 1.7g H, 12.8g C. Sample B has 28.7g Cu, 10.0g O, 0.85g H, 6.4g C. Verify the law.

Solution:
Sample A ratio Cu:O:H:C = 57.5:20.0:1.7:12.8 ≈ 34:12:1:7.5
Sample B ratio Cu:O:H:C = 28.7:10.0:0.85:6.4 ≈ 34:12:1:7.5
Identical ratios confirm definite proportions

Problem 8: Nitrogen Oxides

N₂O has N:O = 28:16. NO₂ has N:O = 14:32. Show they follow multiple proportions.

Solution:
For fixed N mass (14g):
O masses = 8g (N₂O) and 32g (NO₂)
Ratio = 8:32 = 1:4 (simple whole number)

Problem 9: Precipitation Reaction

When 17.0g silver nitrate reacts with 11.7g sodium chloride, 14.35g silver chloride precipitates. Calculate the mass of sodium nitrate solution formed.

Step 1: Write reaction
AgNO₃ + NaCl → AgCl + NaNO₃

Step 2: Apply mass conservation
Total mass reactants = 17.0g + 11.7g = 28.7g
Mass NaNO₃ = 28.7g – 14.35g = 14.35g

Problem 10: Multiple Proportions

Carbon forms two oxides. Oxide A has C:O = 12:16. Oxide B has C:O = 12:32. Show they obey the law.

Solution:
For fixed C mass (12g):
O masses = 16g and 32g
Ratio = 16:32 = 1:2 (simple whole number)

5 Practice Problems (Unsolved)

Problem 1

When 10g calcium carbonate decomposes, it produces 5.6g calcium oxide. What mass of carbon dioxide is released?

Problem 2

Two samples of sodium chloride were analyzed. Sample A had 23g Na and 35.5g Cl. Sample B had 46g Na and 71g Cl. Do they obey definite proportions?

Problem 3

Nitrogen forms two oxides. NO has N:O = 14:16. N₂O₃ has N:O = 28:48. Verify multiple proportions.

Problem 4

1.0g hydrogen reacts with 8.0g oxygen to form water. If 3.0g hydrogen reacts with 20.0g oxygen, what mass of water forms and which reactant is in excess?

Problem 5

Iron forms two chlorides. FeCl₂ has Fe:Cl = 56:71. FeCl₃ has Fe:Cl = 56:106.5. Show they follow the law of multiple proportions.

Ready for Atomic Structure Foundations?

Explore Dalton’s Atomic Theory and its modern modifications

Next: Dalton’s Atomic Theory →

Laws of Chemical Combination with Numerical problems class 11th Chemistry Unit-I

Laws of Chemical Combination (Numerical Problems)

Fundamental Laws Governing Chemical Reactions

1. Law of Conservation of Mass (Lavoisier, 1789)

Key Features:

Example 1: Thermal Decomposition

Example 2: Precipitation Reaction

2. Law of Definite Proportions (Proust, 1799)

Key Features:

Example 1: Carbon Dioxide

Example 2: Copper Carbonate

3. Law of Multiple Proportions (Dalton, 1803)

Key Features:

Example 1: Nitrogen Oxides

Example 2: Sulfur Fluorides

10 Solved Numerical Problems

Problem 1: Conservation of Mass

Problem 2: Definite Proportions

Problem 3: Conservation in Neutralization

Problem 4: Hydrate Composition

Problem 5: Carbon Oxides Ratio

Problem 6: Combustion Analysis

Problem 7: Mineral Composition

Problem 8: Nitrogen Oxides

Problem 9: Precipitation Reaction

Problem 10: Multiple Proportions

5 Practice Problems (Unsolved)

Problem 1

Problem 2

Problem 3

Problem 4

Problem 5

Ready for Atomic Structure Foundations?

1 thought on “Laws of Chemical Combination with Numerical problems class 11th Chemistry Unit-I”

Laws of Chemical Combination (Numerical Problems)

Fundamental Laws Governing Chemical Reactions

1. Law of Conservation of Mass (Lavoisier, 1789)

Key Features:

Example 1: Thermal Decomposition

Example 2: Precipitation Reaction

2. Law of Definite Proportions (Proust, 1799)

Key Features:

Example 1: Carbon Dioxide

Example 2: Copper Carbonate

3. Law of Multiple Proportions (Dalton, 1803)

Key Features:

Example 1: Nitrogen Oxides

Example 2: Sulfur Fluorides

10 Solved Numerical Problems

Problem 1: Conservation of Mass

Problem 2: Definite Proportions

Problem 3: Conservation in Neutralization

Problem 4: Hydrate Composition

Problem 5: Carbon Oxides Ratio

Problem 6: Combustion Analysis

Problem 7: Mineral Composition

Problem 8: Nitrogen Oxides

Problem 9: Precipitation Reaction

Problem 10: Multiple Proportions

5 Practice Problems (Unsolved)

Problem 1

Problem 2

Problem 3

Problem 4

Problem 5

Ready for Atomic Structure Foundations?

Related Posts

1 thought on “Laws of Chemical Combination with Numerical problems class 11th Chemistry Unit-I”