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Atoms and Molecules - Laws of Chemical Combination

Grade 9CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Laws of Chemical Combination were established by Antoine L. Lavoisier and Joseph L. Proust to explain the nature of chemical reactions.

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Law of Conservation of Mass: Mass can neither be created nor destroyed in a chemical reaction. This means the total mass of the reactants must equal the total mass of the products (Massreactants=MassproductsMass_{reactants} = Mass_{products}).

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Law of Constant Proportions: Also known as the Law of Definite Proportions, it states that in a chemical substance, the elements are always present in definite proportions by mass, regardless of the source or method of preparation.

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For example, in water (H2OH_2O), the ratio of the mass of hydrogen to the mass of oxygen is always 1:81:8. Similarly, in ammonia (NH3NH_3), nitrogen and hydrogen are always present in the ratio 14:314:3 by mass.

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Dalton's Atomic Theory: John Dalton provided the theoretical proof for these laws. He postulated that all matter is made of tiny, indivisible particles called atoms, and that atoms of different elements combine in simple whole-number ratios to form compounds.

📐Formulae

Total Massreactants=Total MassproductsTotal\ Mass_{reactants} = Total\ Mass_{products}

Ratio of Mass (Water)  ⟹  H:O=1:8\text{Ratio of Mass (Water)} \implies H:O = 1:8

Ratio of Mass (Ammonia)  ⟹  N:H=14:3\text{Ratio of Mass (Ammonia)} \implies N:H = 14:3

Percentage of an element=Mass of element in compoundTotal molar mass of compound×100\text{Percentage of an element} = \frac{\text{Mass of element in compound}}{\text{Total molar mass of compound}} \times 100

💡Examples

Problem 1:

In a reaction, 5.3 g5.3\text{ g} of sodium carbonate reacted with 6.0 g6.0\text{ g} of ethanoic acid. The products were 2.2 g2.2\text{ g} of carbon dioxide, 0.9 g0.9\text{ g} of water and 8.2 g8.2\text{ g} of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.

Solution:

Total mass of reactants = 5.3 g+6.0 g=11.3 g5.3\text{ g} + 6.0\text{ g} = 11.3\text{ g}. Total mass of products = 8.2 g(sodium ethanoate)+2.2 g(CO2)+0.9 g(H2O)=11.3 g8.2\text{ g} (\text{sodium ethanoate}) + 2.2\text{ g} (CO_2) + 0.9\text{ g} (H_2O) = 11.3\text{ g}.

Explanation:

Since the Total Massreactants=Total Massproducts=11.3 gTotal\ Mass_{reactants} = Total\ Mass_{products} = 11.3\text{ g}, the law of conservation of mass is verified.

Problem 2:

Hydrogen and oxygen combine in the ratio of 1:81:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g3\text{ g} of hydrogen gas?

Solution:

Mass of Oxygen required = 3 g×8=24 g3\text{ g} \times 8 = 24\text{ g}.

Explanation:

According to the Law of Constant Proportions, 1 g1\text{ g} of H2H_2 requires 8 g8\text{ g} of O2O_2. Therefore, 3 g3\text{ g} of H2H_2 will require 3×8=24 g3 \times 8 = 24\text{ g} of O2O_2.

Problem 3:

3.0 g3.0\text{ g} of carbon is burnt in 8.00 g8.00\text{ g} oxygen, 11.00 g11.00\text{ g} of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g3.00\text{ g} of carbon is burnt in 50.00 g50.00\text{ g} of oxygen?

Solution:

The mass of carbon dioxide formed will still be 11.00 g11.00\text{ g}.

Explanation:

This is governed by the Law of Constant Proportions. Carbon and Oxygen always combine in a fixed ratio of 3:83:8 by mass to form CO2CO_2. Even if 50 g50\text{ g} of oxygen is available, only 8 g8\text{ g} will react with 3 g3\text{ g} of carbon, leaving 42 g42\text{ g} of oxygen unreacted.