Atoms and Molecules - Formula Sheet

Key Formulas for Atoms and Molecules

1. Number of Moles of a Substance

There are multiple ways to calculate the number of moles depending on the given data:

• From Mass: Number of Moles = Mass of the substance / Gram Atomic (or Molecular) Mass
• From Number of Particles: Number of Moles = Number of atoms or molecules / (6.023 times 10 to the power of 23)
• From Gas Volume (at STP): Number of Moles = Given volume of gas at STP / 22.4 Litres (or Given volume in ml / 22400 ml)

Symbols Defined:

• Mass of the substance: The actual given mass of the element or compound in grams (g).
• Gram Atomic (or Molecular) Mass: The mass of one mole of atoms or molecules in grams (g/mol).
• 6.023 times 10 to the power of 23: Avogadro's number (constant representing particles per mole).
• Given volume of gas: The measured volume of the gaseous substance under standard temperature and pressure (STP).
• 22.4 Litres (22400 ml): The standard molar volume of any ideal gas at STP.

2. Average Atomic Mass Calculation

Average Atomic Mass = (Mass of Isotope 1 times Percentage Abundance 1) + (Mass of Isotope 2 times Percentage Abundance 2) + ...

Symbols Defined:

• Mass of Isotope: The atomic mass of a specific isotope in atomic mass units (amu).
• Percentage Abundance: The natural fractional abundance of the isotope on Earth (for example, 20 percent abundance is written as 0.20 in calculation).

3. Relation Between Relative Molecular Mass and Vapour Density

Relative Molecular Mass = 2 times Vapour Density

Symbols Defined:

• Relative Molecular Mass (RMM): The ratio of the mass of a molecule to one-twelfth of the mass of a carbon-12 atom (dimensionless).
• Vapour Density (VD): The ratio of the mass of a certain volume of gas to the mass of an equal volume of hydrogen under identical temperature and pressure (dimensionless).

4. Percentage Composition of an Element in a Compound

Percentage of Element = (Mass of that element in 1 mole of the compound / Gram Molecular Mass of the compound) times 100

Symbols Defined:

• Mass of that element: Total mass contributed by the specific element in one mole of the chemical formula (atomic mass of element times its atomicity).
• Gram Molecular Mass of the compound: The total molar mass of the entire compound (g/mol).

5. Calculation of Atomicity

Atomicity of a Homoatomic Element = Molecular Mass / Atomic Mass

Symbols Defined:

• Molecular Mass: The relative molecular mass of the element's molecule.
• Atomic Mass: The relative atomic mass of the individual atom of the element.
• Atomicity: The total number of atoms constituting the molecule.