Mole Concept and Stoichiometry Short notes and Selected PYQs

Atomic Mass: Mass of one atom of an element expressed in atomic mass units (u or amu). 1 u = 1/12 mass of C-12 atom.
Relative Atomic Mass: Ratio of average mass of atom to 1/12 mass of C-12. Dimensionless.
Molecular Mass: Sum of atomic masses of all atoms in a molecule. Example: H₂O = 2(1) + 16 = 18 u.
Formula Mass: For ionic compounds. Example: NaCl = 23 + 35.5 = 58.5 u.
Gram Atomic Mass: Atomic mass expressed in grams. Example: C has atomic mass 12 u; gram atomic mass = 12 g.
Gram Molecular Mass: Molecular mass expressed in grams. Example: H₂O has molecular mass 18 u; gram molecular mass = 18 g.

Mole: SI unit for amount of substance. One mole contains Avogadro's number of particles.
Avogadro's Number (Nₐ): 6.022 × 10²³ particles/mole. Can be atoms, molecules, ions, or electrons.
Molar Mass: Mass of one mole of substance (in grams). Numerically equal to atomic/molecular mass.
Relationship: 1 mole = 6.022 × 10²³ particles = Gram atomic/molecular mass.
Example: 1 mole of C = 12 g = 6.022 × 10²³ atoms. 1 mole of H₂O = 18 g = 6.022 × 10²³ molecules.
Moles Calculation: Number of moles (n) = Given mass / Molar mass = Number of particles / Avogadro's number.

Molar Volume: Volume occupied by one mole of any gas at STP (Standard Temperature and Pressure).
STP Conditions: Temperature = 0°C (273 K), Pressure = 1 atm (760 mm Hg).
Molar Volume at STP: 22.4 liters or 22,400 mL for any gas.
Avogadro's Law: Equal volumes of all gases at same T and P contain equal number of molecules.
Gay-Lussac's Law: Gases react in simple whole number ratios by volume at constant T and P.
Example: H₂ + Cl₂ → 2HCl. Volume ratio: 1:1:2 (one volume H₂ reacts with one volume Cl₂ to give two volumes HCl).

Empirical Formula: Simplest whole number ratio of atoms in a compound. Example: CH for benzene.
Molecular Formula: Actual number of atoms of each element in one molecule. Example: C₆H₆ for benzene.
Relationship: Molecular formula = n × (Empirical formula), where n is a whole number.
Calculation Steps: (1) Find mass % of elements. (2) Convert to moles. (3) Find simplest ratio. (4) Determine molecular formula if molecular mass given.
Example: Empirical formula CH₂O, molecular mass 180. Empirical formula mass = 30. n = 180/30 = 6. Molecular formula = C₆H₁₂O₆.

Stoichiometry: Quantitative study of reactants and products in chemical reactions.
Balanced Equation: Shows mole ratios. Example: 2H₂ + O₂ → 2H₂O (2 moles H₂ react with 1 mole O₂).
Mole Ratio: Coefficient ratio in balanced equation. Used to calculate amounts of reactants/products.
Mass-Mass Calculations: Use mole ratio to convert mass of reactant to mass of product.
Mass-Volume Calculations: Convert mass to moles, use mole ratio, convert to volume (if gas at STP).
Limiting Reagent: Reactant that is completely consumed first, limiting amount of product formed.

Percentage Composition: % of element = (Mass of element in compound / Molecular mass) × 100.
Example: H₂O has 2 H (mass 2) and 1 O (mass 16). % H = (2/18) × 100 = 11.11%. % O = (16/18) × 100 = 88.89%.
Water of Crystallization: Calculate from mass loss on heating. Example: CuSO₄·5H₂O → CuSO₄ + 5H₂O.
Yield Calculations: % Yield = (Actual yield / Theoretical yield) × 100.
Purity: % Purity = (Mass of pure substance / Total mass) × 100.
Applications: Used in industry to calculate raw material requirements and product yields.

Mole Concept and Stoichiometry