Questions
5–8 questions in board/competitive papers
Difficulty
Medium
Importance
Core — never skip
Overview
Atoms and Molecules forms the fundamental basis of chemistry, governing how matter interacts through fixed mass ratios and discrete particle units. Mastery of this topic is essential as it serves as the prerequisite for stoichiometry, thermodynamics, and electrochemistry in competitive board and PSU entrance exams.
Laws of Chemical Combination
These laws define the quantitative relationships in chemical reactions, establishing that mass is conserved and compounds contain elements in fixed proportions. Understanding these principles is critical for solving empirical formula problems and balancing complex chemical equations.
- Law of Conservation of Mass: Mass can neither be created nor destroyed in a chemical reaction
- Law of Constant Proportions: Elements in a chemical compound are always present in definite proportions by mass
- Dalton's Atomic Theory: Atoms are indivisible, indestructible building blocks of matter
- Law of Multiple Proportions: If two elements form more than one compound, the ratios of masses are small whole numbers
Atomic and Molecular Mass
Atomic mass is measured relative to 1/12th the mass of a Carbon-12 atom, representing the average mass of isotopes. Molecular mass is the sum of atomic masses of all atoms present in a single molecule, vital for calculating molar mass in numericals.
- Atomic Mass Unit (u): 1/12th mass of one C-12 atom
- Formula Mass: Used specifically for ionic compounds like NaCl
- Gram Atomic Mass: Atomic mass expressed in grams representing 1 mole
- Relative Molecular Mass = (Mass of 1 molecule) / (1/12th mass of C-12 atom)
The Mole Concept
The mole acts as a bridge between the microscopic world of atoms and the macroscopic world of laboratory measurements. Converting between mass, volume of gas at STP, and number of particles is the most frequently tested calculation type.
- 1 Mole = 6.022 x 10^23 particles (Avogadro's Number)
- Molar Volume of gas at STP = 22.4 Liters
- Number of Moles (n) = Given Mass (m) / Molar Mass (M)
- Number of Particles = n x Avogadro's Number
Formula Sheet
n = m / M
n = N / NA
n = Volume (L) / 22.4 L (at STP)
Mass = n x M
Exam Tip
Always convert mass to moles first before attempting any stoichiometry conversion—it is the universal 'gateway' unit for 90% of chemistry numericals.
Common Mistakes
- Confusing Atomic Mass (u) with Molar Mass (g/mol) during calculations
- Forgetting to multiply the atomic mass by the subscript in a molecular formula (e.g., using 16 instead of 32 for O2)
- Miscalculating the number of atoms in a formula by ignoring stoichiometry coefficients
More Revision Notes
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