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Mole Concept

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Topics

  • Concept of Mole
  • Quantities Related on Mole Concept and Their Formula
  • Experiment

Concept of Mole:

In a chemical reaction, the quantity of a substance can be expressed in terms of its mass or the number of atoms/molecules. Chemical equations indicate the number of atoms or molecules participating in a reaction, making it more convenient to refer to substances in terms of the number of molecules or atoms rather than mass.

  • The term “mole” was introduced in 1896 by Wilhelm Ostwald, derived from the Latin word moles, meaning a "heap" or "pile," to represent a collection of atoms or molecules.
  • A mole is defined as the amount of a substance that contains exactly 6.02214076×1023 elementary entities (atoms, molecules, or ions).
  • In 1967, the SI officially adopted the unit mole (mol) for the amount of substance.
  • A mole provides a link between the mass of a substance (in grams) and the number of atoms or molecules it contains.

1 mole = 6.022×1023 entities = relative mass in grams.

The mole simplifies the counting of atoms and molecules in chemical reactions, making it an essential unit in chemistry.

Number of moles of a substances (n) = `"Mass of substance in grams"/"Molecular mass of substance"`

To explain the concept of a mole, consider the formation of water (H₂O):

2H₂+O₂→2H₂O​​​

  • Two molecules of hydrogen (H₂) react with one molecule of oxygen (O2) to form two molecules of water (H₂O).
  • In terms of mass, 4 u (atomic mass units) of hydrogen combine with 32 u of oxygen to form 36 u of water.
  • One mole of H₂ contains 6.022×1023 molecules of hydrogen.
  • One mole of O₂ contains 6.022×1023 molecules of oxygen.
  • When two moles of H₂ react with one mole of O₂, they form two moles of H₂O, or 2×6.022×1023 water molecules.

Quantities Related on Mole Concept and Their Formula:

1. Atomic Mass: The atomic mass of an element represents the mass of one atom in atomic mass units (u).

For example,

  • Hydrogen: 1 u (1 atom of hydrogen weighs 1 u).
  • Oxygen: 16 u (1 atom of oxygen weighs 16 u).

2. Gram Atomic Mass (Molar Mass of Atoms): The gram atomic mass is the mass of 1 mole of atoms of an element. It has the same numerical value as the atomic mass but is expressed in grams.

For example,

  • Hydrogen: Atomic mass = 1 u, gram atomic mass = 1 g (1 g of hydrogen contains 6.022×1023.
  • Oxygen: Atomic mass = 16 u, gram atomic mass = 16 g (16 g of oxygen contains 6.022×1023.

3. Gram Molecular Mass (Molar Mass of Molecules): The gram molecular mass is the mass of 1 mole of molecules of a substance. It has the same numerical value as the molecular mass but is expressed in grams.

For example,

Water (H₂O): Molecular mass = 18 u, gram molecular mass = 18 g (18 g of water contains 6.022×1023.

Experiment

1. Aim: To understand the relationship between the mass and the number of grains in different types of dal (Tur, Masoor, Chana) and to explore the concept of equal numbers and masses.

2. Requirements: tur dal, masoor dal, chana dal, weighing balance, paper for sketching, glue or paint.

3. Procedure

  • Using a balance, weigh a single grain of tur dal, masoor dal, and chana dal and record their masses.
  • Measure 10 g of each type of dal separately. Count the number of grains in each 10 g portion and compare whether the numbers differ.
  • Draw a line sketch on a piece of paper.
  • Paste an equal number of grains of tur dal, masoor dal, and chana dal in sequence.
  • Count the grains used, calculate how many dozens were needed for each type, and determine the mass of grains used for each dal type.

4. Conclusion: The mass of a single grain varies for each dal type, with heavier grains (e.g., Chana dal) having fewer grains in a given mass compared to lighter grains (e.g., Masoor dal). Equal numbers of grains of different dals result in different total masses due to differences in individual grain masses. Similarly, in chemical reactions, the number of atoms or molecules is more significant than just their mass, aiding in accurate measurement and interpretation.

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