Law of Chemical Combination in Mole Concept

Introduction

The Laws of Chemical Combination describe how elements combine to form compounds. Using the mole concept, these laws can be precisely understood by connecting mass, moles, and atoms.

1. Law of Conservation of Mass

Mass is neither created nor destroyed in a chemical reaction. In terms of moles, the total number of moles of atoms in reactants equals that in products — atoms rearrange but total mass stays constant.

2. Law of Definite Proportions (Constant Composition)

A chemical compound always contains the same elements in the same proportion by mass. The mole ratio of elements in a compound is fixed. For example, water always has hydrogen and oxygen in a mole ratio of 2:1, corresponding to its formula H2O.

3. Law of Multiple Proportions

When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in small whole number ratios. Different compounds have mole ratios that are simple multiples of each other. For example, carbon and oxygen form CO and CO2, with oxygen atoms combining in 1:2 ratio.

Application of Mole Concept

The mole concept bridges the microscopic and macroscopic worlds, allowing chemists to count atoms and molecules quantitatively. It helps determine stoichiometric ratios and verify these fundamental chemical laws.

Example: Verifying Law of Definite Proportions

Water always contains 2 moles of hydrogen for every 1 mole of oxygen.

Mass ratio calculation:
Hydrogen: 2 mol × 1 g/mol = 2 g
Oxygen: 1 mol × 16 g/mol = 16 g
Mass ratio = 2 / 16 = 1 / 8
This fixed ratio confirms the law of definite proportions.

The Law of Chemical Combination explained through the mole concept provides a clear understanding of how matter behaves quantitatively in reactions.

Multiple Choice Questions (MCQs)

1. Which law states that mass is conserved during a chemical reaction?
   A. Law of Definite Proportions
   B. Law of Conservation of Mass ✅
   C. Law of Multiple Proportions
   D. Avogadro’s Law
2. Water has a hydrogen to oxygen mole ratio of:
   A. 1:1
   B. 2:1 ✅
   C. 8:1
   D. 1:2
3. CO and CO₂ illustrate which law?
   A. Law of Reciprocal Proportions
   B. Law of Multiple Proportions ✅
   C. Avogadro’s Law
   D. Law of Constant Volume

True or False

• The total number of atoms changes in a chemical reaction. – ❌ False (Atoms rearrange, total number stays the same)
• The Law of Definite Proportions holds for all pure compounds. – ✅ True
• Mole concept is unrelated to stoichiometry. – ❌ False (It is foundational to stoichiometry)

Quick Quiz

Question: If 4 grams of hydrogen combines with 32 grams of oxygen, what compound is likely formed?

Answer: H₂O (Water) — Mass ratio is 1:8, consistent with Law of Definite Proportions.

Canonical: https://chemash.in/law-of-chemical-combination-mole-concept