Finding Number of Moles Using Mass, Volume & Particles
By Abhishek Sengar | CHEMCA – JEE & NEET Chemistry
Recommended Prerequisite
Before manipulating particles, mass, and volume, ensure you understand the core mole formulas. Check out our foundational guide: Some Basic Concepts of Chemistry for Class 11 and JEE/NEET.
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Lecture 3: Some Basic Concepts of Chemistry
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Hello students! Welcome to Lecture 3 of Some Basic Concepts of Chemistry. In previous lectures, we learned the formulas to find moles. Today, we are putting them to work! We will learn how to interconvert Mass, Volume, Moles, Molecules, and individual Atoms.
1. The 4 Golden Steps for Particle Calculations
Whenever you are asked to find the number of molecules or atoms from a given mass or volume, follow this strict, foolproof sequence:
Step 1: Find the Number of Moles
Convert the given information (mass, volume, etc.) into moles first.
Step 2: Find the Number of Molecules
Multiply the moles by Avogadro's Number (NA = 6.022 × 1023).
Step 3: Find Total Atoms
Multiply the total number of molecules by the total atoms in one molecule.
Step 4: Find Specific Atoms
Multiply the total number of molecules by the number of specific atoms in one molecule.
2. Forward Calculation: Mass ➔ Particles
Problem: In 196g of H2SO4, find moles, molecules, total atoms, and Oxygen atoms.
- Molar Mass of H2SO4: 2(1) + 32 + 4(16) = 98 g/mol
- Number of Moles: Mass / Molar Mass = 196 / 98 = 2 Moles
- Number of Molecules: Moles × NA = 2NA
- Total Atoms: One H2SO4 has 7 atoms (2H + 1S + 4O). Total atoms = 7 × 2NA = 14NA
- Oxygen Atoms: One H2SO4 has 4 Oxygen atoms. Total Oxygen = 4 × 2NA = 8NA
3. Reverse Calculation: Particles ➔ Mass
If you are given the number of particles, you must divide by Avogadro's Number (NA) to get the number of moles first, then multiply by molar mass.
Problem: Find the mass of 3.011 × 1025 molecules of H2SO4.
- Moles: (3.011 × 1025) / (6.022 × 1023) = 0.5 × 102 = 50 moles.
- Mass: Moles × Molar Mass = 50 × 98 = 4900g (or 4.9 kg).
4. Working with Gas Volumes
For gases at Old STP (1 atm, 273.15K), 1 mole occupies 22.4 Liters. Like everything else, route your calculations through moles!
Volume ➔ Mass
Mass of 11.2L of CO2 at STP:
Moles = 11.2 / 22.4 = 0.5 moles.
Mass = 0.5 × 44 = 22g.
Mass ➔ Volume
Volume of 11g of CO2 at STP:
Moles = 11 / 44 = 0.25 moles.
Volume = 0.25 × 22.4 = 5.6L.
5. Crucial Vocabulary Warning
In physical chemistry exams, you will often see archaic but important terminology:
- "1 Gram Atom" = 1 MOLE of Atoms (Not 1 gram mass!)
- "1 Gram Molecule" = 1 MOLE of Molecules
However, if it says "1 gram of atoms", that means the physical mass is 1 gram.
Test Your Understanding! 🎯
Take this 10-question MCQ quiz to verify your grasp of Lecture 3. Explanations and study recommendations will be revealed upon submission.
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