Relative Formula Mass
Science: KS3, topic 2.6.5
Relative Formula Mass
Relative atomic mass
- Relative atomic mass is the average mass of an atom.
- The symbol for relative atomic mass is Ar.
- Every element has its own relative atomic mass.
- You can find relative atomic masses on the periodic table.
Relative formula mass
- Relative formula mass adds up relative atomic masses.
- The masses come from every atom in the formula.
- The symbol for relative formula mass is Mr.
- Relative formula mass is also called relative molecular mass.
The Mr equation
- The equation for relative formula mass is Mr = {\Sigma}Ar.
- The symbol {\Sigma} means 'add together'.
- To find Mr, add up the relative atomic mass of every atom in the formula.
Worked example
- Water has the formula H2O.
- Water contains two hydrogen atoms and one oxygen atom.
- Hydrogen has a relative atomic mass of 1.
- Oxygen has a relative atomic mass of 16.
- The relative formula mass of water is 1 + 1 + 16 = 18.
Test yourself on Relative Formula Mass
In the equation Mr = ΣAr, what does the symbol Σ tell you to do?
Select the correct statement in each pair.
Name the two elements that make up water.
Water (H2O) contains hydrogen atoms with a relative atomic mass of 1 and an oxygen atom with a relative atomic mass of 16. Which atom has the greater relative atomic mass?
A compound's formula contains only one atom. How does its relative formula mass compare with that atom's relative atomic mass?