Relative Atomic and Formula Mass
Simple Explanation
Relative atomic mass (Ar) is the average mass of an atom of an element, measured relative to 1/12th the mass of a carbon-12 atom. Relative formula mass (Mr) is simply the sum of the relative atomic masses of every atom in a compound's formula.
Why Do We Need It?
Every calculation about how much of a substance you have — moles, concentration, yield — starts from knowing its relative formula mass.
Worked Example
Find the relative formula mass of water
Find the relative formula mass (Mr) of water, H₂O. (Ar: H = 1, O = 16)
Why Does This Work?
Because a formula tells you exactly how many of each atom are bonded together, adding up their individual masses gives the total mass of the whole molecule, relative to the same carbon-12 standard.
Real-Life Example
Reading a fertilizer label
Fertilizer bags list the percentage of nitrogen, phosphorus and potassium by mass.
Calculating those percentages starts from knowing the relative formula mass of each compound in the fertilizer.
Practice
Find the relative formula mass of carbon dioxide, CO₂. (Ar: C = 12, O = 16)
EasyCommon mistake
Forgetting to multiply by the number of atoms of each element — e.g. treating CO₂ as if it only had one oxygen atom.
Quick Review
- Ar = relative atomic mass of one element.
- Mr = relative formula mass = sum of all atoms' Ar values in the formula.
- Always check the subscript numbers in a formula carefully.