Relative Formula Mass (OCR GCSE Chemistry A (Gateway Science Suite)): Revision Notes
3.1.2 Relative Formula Mass
What is "Relative Formula Mass"?
Atoms are incredibly tiny and have very little mass. Because of this, scientists use a special way to measure their mass called relative atomic mass (symbol: Ar).
The relative atomic mass of an element compares the mass of its atoms to the mass of a carbon-12 atom, which is used as a standard. The Ar values for different elements can be found on the periodic table. These values are just numbers and don't have any units.
Calculating Relative Formula Mass (Mr):
When we talk about the mass of a compound (a substance made of more than one element), we use something called the relative formula mass, which has the symbol Mr.
To calculate the Mr of a compound, follow these steps:
- Find the Formula: Look at the chemical formula of the compound to see how many atoms of each element are present.
- Use Ar Values: Use the Ar values from the periodic table for each element in the compound.
- Add Them Up: Multiply the Ar value by the number of atoms of that element in the formula, then add all these values together to get the Mr.
Example: Carbon Dioxide (CO₂):
- Let's calculate the Mr for carbon dioxide (CO₂):
- The formula CO₂ tells us there is 1 carbon atom and 2 oxygen atoms.
- The Ar value for carbon (C) is 12, and the Ar value for oxygen (O) is 16.
- Now, add them up: Mr of CO2 =
- So, the Mr of CO₂ is 44.
Relative Formula Mass of Ionic Compounds: Compounds like sodium chloride (NaCl) don't exist as individual molecules but as a lattice of ions. However, you can still calculate the Mr in the same way, using the numbers in the formula.
Key Points to Remember:
- Ar is the relative atomic mass of an element.
- Mr is the relative formula mass of a compound, and it's calculated by adding up the Ar values of all the atoms in the formula.
- Both Ar and Mr are just numbers with no units because they are relative.