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Some Basic Concepts of Chemistry - Laws of Chemical Combination

Grade 11CBSEChemistry

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. Matter can neither be created nor destroyed (mreactants=mproductsm_{reactants} = m_{products}).

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Law of Definite Proportions: 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 the ratio of small whole numbers.

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Gay Lussac’s Law of Gaseous Volumes: When gases combine or are produced in a chemical reaction, they do so in a simple ratio by volume, provided all gases are at the same temperature and pressure.

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Avogadro’s Law: Proposed by Amedeo Avogadro, it states that equal volumes of all gases at the same temperature and pressure should contain an equal number of molecules (V∝nV \propto n).

📐Formulae

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

Ratio of masses in Law of Multiple Proportions=a:b (where a,b∈Z+)\text{Ratio of masses in Law of Multiple Proportions} = a : b \text{ (where } a, b \in \mathbb{Z}^+ \text{)}

V1n1=V2n2 (At constant T and P)\frac{V_1}{n_1} = \frac{V_2}{n_2} \text{ (At constant } T \text{ and } P \text{)}

\text{Mass % of an element} = \frac{\text{Mass of that element in the compound}}{\text{Molar mass of the compound}} \times 100

💡Examples

Problem 1:

Carbon combines with Oxygen to form two compounds, COCO and CO2CO_2. In COCO, 12 g12\text{ g} of Carbon reacts with 16 g16\text{ g} of Oxygen. In CO2CO_2, 12 g12\text{ g} of Carbon reacts with 32 g32\text{ g} of Oxygen. Show that these data illustrate the Law of Multiple Proportions.

Solution:

  1. In COCO, mass of Oxygen that combines with 12 g12\text{ g} of Carbon = 16 g16\text{ g}.
  2. In CO2CO_2, mass of Oxygen that combines with 12 g12\text{ g} of Carbon = 32 g32\text{ g}.
  3. The mass of Carbon is fixed at 12 g12\text{ g} in both cases.
  4. The ratio of the masses of Oxygen is 16:3216 : 32, which simplifies to 1:21 : 2.

Explanation:

Since 1:21:2 is a simple whole-number ratio, it confirms the Law of Multiple Proportions, which states that when one element's mass is fixed, the other element's masses will be in a simple ratio.

Problem 2:

100 mL100\text{ mL} of Hydrogen gas (H2H_2) reacts with 50 mL50\text{ mL} of Oxygen gas (O2O_2) to form water vapor (H2OH_2O). What is the volume of water vapor produced at the same temperature and pressure, and which law does this follow?

Solution:

The balanced chemical equation is: 2H2(g)+O2(g)→2H2O(g)2H_{2(g)} + O_{2(g)} \rightarrow 2H_2O_{(g)} According to the stoichiometry and Gay Lussac's Law: 22 volumes of H2H_2 react with 11 volume of O2O_2 to produce 22 volumes of H2OH_2O. Given VH2=100 mLV_{H_2} = 100\text{ mL} and VO2=50 mLV_{O_2} = 50\text{ mL}: Volume of H2OH_2O produced = 100 mL100\text{ mL}.

Explanation:

This follows Gay Lussac's Law of Gaseous Volumes, which states that the volumes of reactants and products (if gaseous) are in a simple whole-number ratio (2:1:22:1:2 in this case).

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