Relative formula mass (AQA GCSE Chemistry): Revision Notes
Relative formula mass
What is relative formula mass?
Relative formula mass tells us how heavy a compound is compared to other substances. It's the total of all the atomic masses in a chemical formula.
The symbol for relative formula mass is Mr. This is just a number with no units.
The "r" in Mr stands for "relative" - we're comparing the mass to a standard (carbon-12). Since it's a ratio, it has no units like grammes or kilogrammes.
To find the relative formula mass, you need to:
- Look at the chemical formula
- Find the relative atomic mass for each element
- Add them all up
Finding atomic masses
You'll usually get the atomic masses in your exam. They appear on the periodic table too. Here are some common ones you should remember:
Common Relative Atomic Masses:
- Hydrogen (H) = 1
- Carbon (C) = 12
- Nitrogen (N) = 14
- Oxygen (O) = 16
- Sodium (Na) = 23
- Magnesium (Mg) = 24
Simple calculations
Let's start with some straightforward examples to build your confidence.
Worked Example: Oxygen Gas (O₂)
Oxygen gas has 2 oxygen atoms.
Step 1: Identify the atoms present
- 2 oxygen atoms
Step 2: Find the relative atomic mass
- Each oxygen atom has mass = 16
Step 3: Calculate the total
- Total mass = 16 + 16 = 32
Answer: Mr of O₂ = 32
Worked Example: Water (H₂O)
Water has 2 hydrogen atoms and 1 oxygen atom.
Step 1: Break down the formula
- 2 hydrogen atoms
- 1 oxygen atom
Step 2: Calculate each element's contribution
- Hydrogen: 2 × 1 = 2
- Oxygen: 1 × 16 = 16
Step 3: Add them together
- Total: 2 + 16 = 18
Answer: Mr of H₂O = 18
Working with more complex compounds
As compounds become more complex, it's important to work systematically through each element.
Worked Example: Calcium Carbonate (CaCO₃)
This compound contains 1 calcium, 1 carbon, and 3 oxygen atoms.
Step 1: Identify all atoms
- 1 calcium atom
- 1 carbon atom
- 3 oxygen atoms
Step 2: Calculate each element's mass contribution
- Calcium: 1 × 40 = 40
- Carbon: 1 × 12 = 12
- Oxygen: 3 × 16 = 48
Step 3: Add up the total
- Total: 40 + 12 + 48 = 100
Answer: Mr of CaCO₃ = 100
Compounds with brackets like Mg(OH)₂
Critical Concept: Understanding Brackets
When you see brackets in a chemical formula, you must multiply everything inside the brackets by the number outside. This is one of the most common mistakes students make!
Worked Example: Magnesium Hydroxide Mg(OH)₂
Step 1: Understand what the brackets mean
- The subscript ₂ outside the brackets means there are 2 of everything inside
- So Mg(OH)₂ contains:
- 1 magnesium atom
- 2 oxygen atoms (1 × 2 = 2)
- 2 hydrogen atoms (1 × 2 = 2)
Step 2: Calculate each element's contribution
- Magnesium: 1 × 24 = 24
- Oxygen: 2 × 16 = 32
- Hydrogen: 2 × 1 = 2
Step 3: Add up the total
- Total: 24 + 32 + 2 = 58
Answer: Mr of Mg(OH)₂ = 58
Quick method for brackets
Here's an alternative approach that some students find easier:
Alternative Method: Calculate Brackets First
For Mg(OH)₂, you can work out the mass of what's in brackets first:
Step 1: Calculate the mass of (OH)
- OH has mass = 16 + 1 = 17
Step 2: Apply the multiplier and add the other elements
- Mg(OH)₂ = 24 + (2 × 17) = 24 + 34 = 58
This gives the same answer: Mr of Mg(OH)₂ = 58
Key Points to Remember:
- Relative formula mass (Mr) is just a number with no units
- Add up all the atomic masses in the formula
- For brackets, multiply everything inside by the number outside
- You'll usually be given the atomic masses you need
- Always double-check your arithmetic by adding up carefully
- Work systematically through complex formulas step by step