Molecular Mass
Determines the sum of the atomic masses of all atoms present in a molecule, expressed in atomic mass units (amu).
What is Molecular Mass?
The sum of the atomic masses of all the atoms in one molecule of a substance.
Key formula / rule: Molecular Mass Calculation
Key points
- Define molecular mass.
- Calculate the molecular mass of a given compound.
- Understand the relationship between atomic mass and molecular mass.
- Apply the concept of molecular mass in basic chemical calculations.
Common exam trap
Incorrectly summing atomic masses.
Definitions
- Term
Molecular Mass
- Meaning
The sum of the atomic masses of all the atoms in one molecule of a substance.
- Term
Atomic Mass Unit (amu)
- Meaning
A unit of mass used to express atomic and molecular masses, equal to 1/12th the mass of a carbon-12 atom.
Learning objectives
Define molecular mass.
Calculate the molecular mass of a given compound.
Understand the relationship between atomic mass and molecular mass.
Apply the concept of molecular mass in basic chemical calculations.
Formulae
- Name
Molecular Mass Calculation
- Note
Summation is done for all elements present in the molecule.
- Expression
Molecular Mass = Σ (Atomic Mass of Element × Number of Atoms of Element in Molecule)
Prerequisites
Atomic Mass
Chemical Formulae of Compounds
Periodic Table
Common mistakes
Incorrectly summing atomic masses.
Using incorrect atomic masses (e.g., from memory without checking the periodic table).
Confusing molecular mass with atomic mass or molar mass.
Forgetting to multiply the atomic mass by the number of atoms of that element in the molecule.
Keywords
Molecular Mass
Atomic Mass
amu
Molecule
Chemical Formula
Stoichiometry
Practice preview
The molecular mass of ammonia (NH₃) is 17 amu. If the atomic mass of nitrogen (N) is 14 amu, what is the atomic mass of hydrogen (H)?…
medium
Calculate the molecular mass of methane (CH₄). (Atomic mass of C = 12 amu, H = 1 amu)…
easy
What is the molecular mass of carbon dioxide (CO₂)? (Atomic mass of C = 12 amu, O = 16 amu)…
easy