Mole Concept and Stoichiometry Solved Examples (Class 11 Chemistry)
Master mole calculations, molar mass determinations, and stoichiometric conversions. These problems are fundamental to all quantitative chemistry.
TL;DR: Master mole calculations, molar mass determinations, and stoichiometric conversions. These problems are fundamental to all quantitative chemistry.
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Master mole calculations, molar mass determinations, and stoichiometric conversions. These problems are fundamental to all quantitative chemistry.
Mole Concept and Stoichiometry — Solved Numerical Examples (Step by Step)
Example 1: Calculate the number of moles in 54 g of water (H₂O). (Atomic masses: H=1, O=16)
Solution: Molar mass of H₂O = 2(1) + 16 = 18 g/mol
Number of moles = mass / molar mass = 54 / 18 = 3 mol
Example 2: How many grams of NaCl are needed to prepare 2 moles? (Atomic masses: Na=23, Cl=35.5)
Solution: Molar mass of NaCl = 23 + 35.5 = 58.5 g/mol
Mass = moles × molar mass = 2 × 58.5 = 117 g
Example 3: Calculate the number of atoms in 1 mole of carbon. (Avogadro's number = 6.02 × 10²³)
Solution: 1 mole of any substance contains Avogadro's number of particles
Number of atoms = 1 × 6.02 × 10²³ = 6.02 × 10²³
Example 4: What is the mass of 3 × 10²³ molecules of CO₂? (Atomic masses: C=12, O=16; Avogadro's number = 6 × 10²³)
Solution: Molar mass of CO₂ = 12 + 2(16) = 44 g/mol
Number of moles = (3 × 10²³) / (6 × 10²³) = 0.5 mol
Mass = 0.5 × 44 = 22 g
Example 5: Calculate the percentage composition of carbon in C₆H₁₂O₆ (glucose). (Atomic masses: C=12, H=1, O=16)
Solution: Molar mass of C₆H₁₂O₆ = 6(12) + 12(1) + 6(16) = 72 + 12 + 96 = 180 g/mol
Mass of carbon in molecule = 6 × 12 = 72 g/mol
Percentage of carbon = (72 / 180) × 100 = 40%
Example 6: How many molecules are present in 8 g of oxygen gas (O₂)? (Atomic mass: O=16; Avogadro's number = 6 × 10²³)
Solution: Molar mass of O₂ = 2 × 16 = 32 g/mol
Number of moles = 8 / 32 = 0.25 mol
Number of molecules = 0.25 × 6 × 10²³ = 1.5 × 10²³
Tips
- Molar mass (M) is the mass of 1 mole of a substance in grams; always calculate it from atomic masses.
- Moles = mass (g) / molar mass (g/mol); rearrange as needed for different unknowns.
- Avogadro's number (6.02 × 10²³ or 6 × 10²³) is the number of particles per mole.
Frequently Asked Questions
Why do we use the mole concept in chemistry?
The mole allows us to relate macroscopic quantities (grams) to microscopic quantities (atoms/molecules), making stoichiometric calculations possible.
Is molar mass the same as atomic mass?
No. Atomic mass is for a single atom (in atomic mass units, u). Molar mass is for 1 mole of a substance (in g/mol). Numerically, they are equal but with different units.
More Chemistry Solved Examples
- Mole Concept and Stoichiometry
- Atomic Structure (Numericals)
- Electrochemistry (Numericals)
- Chemical Equations and Balancing
- Thermodynamics
- Solutions - Previous Year Questions
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