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 are better 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. For example, carbon and oxygen form CO and CO2, with oxygen atoms combining in a 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 laws.
Example: Verifying Law of Definite Proportions
Water 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 confirms the law of definite proportions.
The Laws of Chemical Combination explained through the mole concept help us understand how matter behaves in reactions.
Multiple Choice Questions (MCQs)
- 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. Law of Reciprocal Proportions - Water has a fixed ratio of hydrogen and oxygen. This is an example of:
A. Law of Conservation of Energy
B. Law of Constant Volume
C. Law of Definite Proportions ✅
D. None of the above
True or False
- True: CO and CO2 follow the Law of Multiple Proportions. (✔️ Oxygen atoms combine in small whole number ratios)
- False: Mole concept is unrelated to mass calculations. (❌ Mole concept directly relates mass, moles, and atoms)
Quick Quiz
Question: If 18g of water is formed, how many grams of hydrogen and oxygen were used?
Answer: 2g Hydrogen and 16g Oxygen. (Ratio 1:8)
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