Summary
The relative atomic mass of an element is the average mass of its atoms compared to one twelfth of the mass of a carbon-12 atom. The relative molecular mass is the sum of the relative atomic masses of the atoms in a molecule. The relative formula mass is the sum of the relative atomic masses of all atoms in a chemical formula.
- Relative Atomic Mass — average mass of an element's atoms, considering isotopes and their abundances. Example: Relative atomic mass of sodium is 23 amu.
- Relative Molecular Mass — average mass of a molecule, considering isotopes of elements present. Example: Relative molecular mass of water (H₂O) is 18 u.
- Relative Formula Mass — sum of relative atomic masses in a chemical formula. Example: Formula mass of calcium nitrate Ca(NO₃)₂ is 164 u.
Exam Tips
Key Definitions to Remember
- Relative atomic mass is the average mass of an element's atoms compared to carbon-12.
- Relative molecular mass is the sum of atomic masses in a molecule.
- Relative formula mass is the sum of atomic masses in a chemical formula.
Common Confusions
- Confusing relative atomic mass with atomic number.
- Mixing up relative molecular mass and molar mass.
Typical Exam Questions
- What is the relative atomic mass of an element? It is the average mass of the element's atoms compared to carbon-12.
- How do you calculate the relative molecular mass of a compound? Add the relative atomic masses of all atoms in the compound.
- What is the formula mass of calcium nitrate Ca(NO₃)₂? 164 u
What Examiners Usually Test
- Understanding of how to calculate relative atomic, molecular, and formula masses.
- Ability to apply these calculations to different chemical compounds.