Class 10 Chemistry: Chapter 5 - Mole Concept (Part B)

Key Topics

• Relative Molecular Mass or Molecular Weight
• Calculation of Relative Molecular Mass
• Concept of Gram Atomic Mass and Gram Molecular Mass
• Numerical problems related to these concepts

Relative Molecular Mass / Molecular Weight

• Definition: A method of expressing the mass of molecules relative to a standard reference.
• Importance: Necessary for accurate calculations in chemistry.
• Historical Background: Initially, hydrogen was used as the reference due to its lightness.
• Issues with initial method: Discrepancies observed in molecular masses when referenced to hydrogen.
• Solution: In 1961, carbon-12 isotope was chosen as the standard unit (1 AMU).
  • Carbon-12 was divided into 12 equal parts.
  • One part is defined as 1 AMU (approximately 1.6 x 10^-24 grams).

Calculation of Relative Atomic Mass

• Example:
  • Oxygen Atom
    • Actual mass: 2.6565 x 10^-23 grams
    • 1 AMU = 1.6 x 10^-24 grams
    • Calculation: Relative atomic mass of oxygen = [2.6565 x 10^-23 / 1.6 x 10^-24] ≈ 16 AMU

Fractional Atomic Mass

• Concept: Some elements do not have a single atomic mass but rather an average due to isotopes.
• Example:
  • Chlorine
    • Isotope Composition: Cl-35 (75%) and Cl-37 (25%)
    • Average Relative Atomic Mass = (35 * 75% + 37 * 25%) / 100 = 35.5 AMU

Gram Atomic Mass

• Definition: Atomic mass of an element expressed in grams.
• Example:
  • Oxygen: 16 grams = 16 Gram Atomic Mass
  • 1 Gram Atom: The amount of an element that contains Avogadro's number of atoms.
    • 1 Gram Atom of Magnesium: 24 g = 24 * 6.022 x 10^23 atoms

Gram Molecular Mass

• Definition: Mass of one mole of a given molecular substance expressed in grams.
• Examples:
  • H2O (Water)
    • Molecular Mass: 18 AMU
    • 1 Molecule: 18 * 1.66 x 10^-24 grams
    • 1 Mole: 18 * 1.66 x 10^-24 * 6.022 x 10^23 grams = 18 grams

Numerical Examples

1. Copper Sulphate (CuSO4·5H2O)

   • Calculations:
     • Cu = 63.5 AMU
     • S = 32 AMU
     • O (for 4 atoms) = 4 * 16 = 64 AMU
     • H2O (for 5 molecules) = 5 * (2 * 1 + 16) = 90 AMU
   • Total = 63.5 + 32 + 64 + 90 = 249.5 AMU

2. Ammonium Sulphate ((NH4)2SO4)

   • Calculations:
     • 2 * (N = 14 AMU) = 28 AMU
     • 4 * (H = 1 AMU) = 4 AMU
     • S = 32 AMU
     • 4 * (O = 16) = 64 AMU
   • Total = 28 + 4 + 32 + 64 = 128 AMU

Summary

• Relative Atomic and Molecular Masses: Essential for comparing masses of elements and compounds.
• Fractional Atomic Mass: Accounts for the presence of isotopes.
• Gram Atomic and Molecular Mass: Helps in practical applications, converting atomic/molecular mass to grams which align with Avogadro's number.
• Numerical Problems: Practice problems to reinforce understanding.

Conclusion

• Today's Focus: Relative Atomic Mass, Relative Molecular Mass, Gram Atomic Mass, Gram Molecular Mass.
• Important foundation for further chemistry topics.
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