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Some Basic Concepts of Chemistry - Laws of Chemical Combination

Grade 11ICSEChemistry

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

🔑Concepts

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Law of Conservation of Mass: Proposed by Antoine Lavoisier, it states that in any physical or chemical change, the total mass of the products is equal to the total mass of the reactants. No mass is lost or gained: ∑Massreactants=∑Massproducts\sum \text{Mass}_{reactants} = \sum \text{Mass}_{products}.

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Law of Definite Proportions (Constant Composition): Proposed by Joseph Proust, it states that a given chemical compound always contains its component elements in a fixed ratio by mass, regardless of its source or method of preparation. For example, H2OH_2O always contains Hydrogen and Oxygen in a 1:81:8 mass ratio.

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Law of Multiple Proportions: Proposed by John Dalton, it states that if 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 a ratio of small whole numbers. Examples include COCO and CO2CO_2.

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Law of Reciprocal Proportions: Proposed by Jeremias Richter, it states that if two different elements AA and BB combine separately with a fixed mass of a third element CC, the ratio of the masses in which they do so is either the same or a simple multiple of the ratio in which AA and BB combine with each other.

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Gay Lussac’s Law of Gaseous Volumes: When gases react together, they do so in volumes which bear a simple whole-number ratio to one another and to the volumes of the products (if gaseous), provided all gases are measured at the same temperature and pressure.

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Avogadro’s Law: Under the same conditions of temperature and pressure, equal volumes of all gases contain an equal number of molecules (V∝nV \propto n).

📐Formulae

Mass of Reactants=Mass of Products\text{Mass of Reactants} = \text{Mass of Products}

Ratio of volumes in Gay Lussac’s Law=V1:V2:V3=n1:n2:n3\text{Ratio of volumes in Gay Lussac's Law} = V_1 : V_2 : V_3 = n_1 : n_2 : n_3

Avogadro’s Equation: V1n1=V2n2\text{Avogadro's Equation: } \frac{V_1}{n_1} = \frac{V_2}{n_2}

Percentage of an element=(Mass of elementTotal mass of compound)×100\text{Percentage of an element} = \left( \frac{\text{Mass of element}}{\text{Total mass of compound}} \right) \times 100

💡Examples

Problem 1:

Carbon combines with Oxygen to form two oxides, COCO and CO2CO_2. In COCO, the ratio of C:OC:O is 12:1612:16, and in CO2CO_2, it is 12:3212:32. Show that this follows the Law of Multiple Proportions.

Solution:

In COCO, 1212 g of Carbon reacts with 1616 g of Oxygen. In CO2CO_2, 1212 g of Carbon reacts with 3232 g of Oxygen. Keeping the mass of Carbon fixed at 1212 g, the ratio of Oxygen masses is 16:3216:32, which simplifies to 1:21:2.

Explanation:

Since 1:21:2 is a simple whole-number ratio, the Law of Multiple Proportions is verified.

Problem 2:

When 100 mL100 \text{ mL} of H2H_2 gas reacts with 50 mL50 \text{ mL} of O2O_2 gas to form water vapor, what is the volume of H2O(g)H_2O_{(g)} produced, and which law is applied?

Solution:

Reaction: 2H2(g)+O2(g)→2H2O(g)2H_{2(g)} + O_{2(g)} \rightarrow 2H_2O_{(g)}. According to Gay Lussac's Law, the volume ratio is 2:1:22:1:2. Since 100 mL100 \text{ mL} H2H_2 and 50 mL50 \text{ mL} O2O_2 are in a 2:12:1 ratio, they react completely to form 100 mL100 \text{ mL} of H2OH_2O vapor.

Explanation:

The Law of Gaseous Volumes states that the volumes of reactants and products are in a simple whole-number ratio (2:1:22:1:2 in this case).

Problem 3:

4.904.90 g of KClO3KClO_3 when heated produced 1.921.92 g of O2O_2 and 2.982.98 g of KClKCl. Show that these results illustrate the Law of Conservation of Mass.

Solution:

Mass of reactant (KClO3KClO_3) = 4.904.90 g. Total mass of products = Mass of KCl+Mass of O2=2.98 g+1.92 g=4.90 g\text{Mass of } KCl + \text{Mass of } O_2 = 2.98 \text{ g} + 1.92 \text{ g} = 4.90 \text{ g}.

Explanation:

Since the mass of the reactant (4.904.90 g) equals the sum of the masses of the products (4.904.90 g), the Law of Conservation of Mass is satisfied.

Laws of Chemical Combination - Revision Notes & Key Formulas | ICSE Class 11 Chemistry