QUANTIFICATION OF ELEMENTS AND COMPOUNDS
- Jun 6
- 2 min read
Atoms and molecules are even smaller and exist in enormous numbers. Scientists use special quantities such as Relative Atomic Mass, Relative Molecular Mass, Avogadro Constant, and Mole to count them efficiently.
Atomic Mass Unit (amu)
The atomic mass unit is based on : 1/12 of the mass of a Carbon-12 atom.
Relative Atomic Mass (Aᵣ)
Definition :
Relative Atomic Mass (Aᵣ) tells us how many times an atom is heavier than 1/12 of a Carbon-12 atom.
Important :
Relative Atomic Mass has no unit.
Common Relative Atomic Masses
Relative Atomic Mass of H - 1
Relative Atomic Mass of C - 12
Relative Atomic Mass of N - 14
Relative Atomic Mass of O - 16
Memory Tip
H-C-N-O → 1-12-14-16
Relative Molecular Mass (Mᵣ)
Relative Molecular Mass (Mᵣ) tells us how many times a molecule is heavier than 1/12 of a Carbon-12 atom.
Mᵣ = Sum of the Relative Atomic Masses of all atoms in the molecule.
Examples
Water (H₂O)
Mᵣ = (2 × 1) + 16 = 18
Carbon dioxide (CO₂)
Mᵣ = 12 + (2 × 16) = 44
Glucose (C₆H₁₂O₆)
Mᵣ = (6 × 12) + (12 × 1) + (6 × 16) = 180
Important:
Relative Molecular Mass also has no unit.
Avogadro Constant (Nₐ)
Nₐ = 6.022 × 10²³ mol⁻¹
Mole (mol)
What is a Mole?
A mole is the SI unit used to measure the amount of a substance.
Definition :
1 mole contains 6.022 × 10²³ particles (atoms, molecules or ions)
Molar Mass (M)
What is Molar Mass?
The mass of 1 mole of a substance is called its molar mass.
Unit - g mol⁻¹, kg mol⁻¹ (SI Unit)
Examples
Na - 23g mol⁻¹, CO2 - 44g mol⁻¹
Most important formula
Amount of substance = Mass of the Substance / Molar mass of substance
n = m / M
Where,
n is the Number of moles
m is the Mass of substance
M is the Molar mass
One-Minute Revision
Aᵣ → Relative Atomic Mass
Mᵣ → Relative Molecular Mass
Avogadro Constant = 6.022 × 10²³ mol⁻¹
1 Mole = 6.022 × 10²³ particles
Molar Mass = Mass of 1 mole
Formula, n = m / M
