Lecture Notes: Atomic Mass and Molecular Mass

Introduction

• Class: 11 Chemistry Series
• Chapter: Basic Concepts of Chemistry
• Topics: Atomic Mass, Molecular Mass
• Aimed at competitive exams like NEET, JEE

Atomic Mass

• Definition: Mass of one atom of an element.
• Unit: Atomic Mass Unit (AMU) or 'u'
• Example: Atomic mass of
  • Oxygen: 16 AMU
  • Nitrogen: 14 AMU
  • Hydrogen: 1 AMU
  • Carbon: 12 AMU

Historical Context

• Previously, atomic masses were found using a reference element, specifically Carbon-12 isotope.
• Carbon-12 Isotope: Used as a reference because it is abundantly available and most stable.
• Reference Mass: 1/12th mass of a Carbon-12 atom is 1.67 x 10^-24 grams.
• This value is defined as 1 AMU.

Important Definition

• Atomic mass: A number showing how many times a single atom of an element is heavier than 1/12th part of a Carbon-12 atom.
  • Example: Oxygen's atomic mass is 16, meaning a single oxygen atom is 16 times heavier than 1/12th part of a Carbon-12 atom.

Molecular Mass

• Definition: Sum of atomic masses of elements present in a molecule.
• Example: Water (H2O)
  • Molecular mass: 18 AMU
  • Meaning: One molecule of water is 18 times heavier than 1/12th part of a Carbon-12 atom.
• Calculation Method: Add up the atomic masses of all elements in the molecule.
  • Carbon Dioxide (CO2): 12 (Carbon) + 16*2 (Oxygen) = 44 AMU
  • Glucose (C6H12O6): 126 + 112 + 16*6 = 180 AMU

Next Steps

• Upcoming Topics: Average atomic mass calculation, Mole concept, Stoichiometry
• Focus: Important for board and competitive exams; tricks and shortcuts for problem-solving will be discussed.

Homework

• Calculate the molecular mass of C2H5OH.
• Understand the meaning of 23 AMU for Sodium.

Conclusion

• Next Session: Average atomic mass, competitive exam questions.
• Goal: Achieve good scores in both board and competitive exams with focused understanding and practice.