Relative formula mass Revision Notes for AQA GCSE Chemistry Combined Science

Relative formula mass

What is relative formula mass?

Relative formula mass tells us how heavy a compound is compared to other substances. It's like weighing all the atoms in a compound and adding them together.

The symbol for relative formula mass is Mr. Remember - it has

no units

. It's just a number!

To find the relative formula mass, you add up all the relative atomic masses (Ar) of every atom in the formula.

infoNote

Think of relative formula mass as the "molecular weight" of a compound - it's a way to compare how heavy different substances are on an atomic scale.

How to calculate relative formula mass

Follow these simple steps:

Look at the formula - identify all the atoms present
Find the relative atomic masses - use your periodic table or data sheet
Count how many of each atom - don't forget subscripts!
Multiply and add - multiply each Ar by how many atoms, then add everything together

chatImportant

Always double-check your counting! Missing even one atom will give you the wrong answer.

Working with simple molecules

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Worked Example: Water (H₂O)

Step 1: Identify the atoms

2 hydrogen atoms
1 oxygen atom

Step 2: Find atomic masses and calculate

2 hydrogen atoms: $2 \times 1 = 2$
1 oxygen atom: $1 \times 16 = 16$

Step 3: Add them together Mr of H₂O = 2 + 16 = 18

lightbulbExample

Worked Example: Oxygen gas (O₂)

Step 1: Identify the atoms

2 oxygen atoms

Step 2: Calculate

2 oxygen atoms: $2 \times 16 = 32$

Mr of O₂ = 32

Working with more complex compounds

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Worked Example: Calcium carbonate (CaCO₃)

Step 1: Break down the formula

1 calcium atom
1 carbon atom
3 oxygen atoms

Step 2: Calculate each component

1 calcium atom: $1 \times 40 = 40$
1 carbon atom: $1 \times 12 = 12$
3 oxygen atoms: $3 \times 16 = 48$

Step 3: Add everything together Mr of CaCO₃ = 40 + 12 + 48 = 100

Dealing with brackets

When you see brackets like Mg(OH)₂, multiply everything inside the brackets by the number outside.

lightbulbExample

Worked Example: Magnesium hydroxide (Mg(OH)₂)

Step 1: Break down the formula

1 magnesium atom
2 oxygen atoms (from the bracket)
2 hydrogen atoms (from the bracket)

Step 2: Calculate each component

1 magnesium atom: $1 \times 24 = 24$
2 oxygen atoms: $2 \times 16 = 32$
2 hydrogen atoms: $2 \times 1 = 2$

Step 3: Add everything together Mr of Mg(OH)₂ = 24 + 32 + 2 = 58

infoNote

Alternative Method: You can also calculate the Mr of OH first $(16 + 1 = 17)$ , then multiply by 2 $(17 \times 2 = 34)$ , then add the Mg $(24 + 34 = 58)$ . This approach can be faster for complex formulas!

Common relative atomic masses to remember

Here are some key values you'll often use:

H = 1
C = 12
N = 14
O = 16
Na = 23
Mg = 24
Al = 27
S = 32

infoNote

These are the most commonly used values in calculations. While you should always check your data sheet during exams, memorising these will speed up your working significantly.

Summary

bookmarkSummary

Key Points to Remember:

Mr has no units - it's just a number
Add up all the atomic masses of every atom in the formula
Use the periodic table to find relative atomic masses
Don't forget to multiply atoms inside brackets by the number outside
Count carefully - make sure you include every single atom

Relative formula mass (AQA GCSE Chemistry Combined Science): Revision Notes