Percentage composition example (Edexcel GCSE Chemistry): Revision Notes

Percentage composition example

Formula for Percentage Composition

$$\textcolor{red}{\%mass} = \frac{\textcolor{red}{\text{relative atomic mass} \times \text{number of atoms} \times 100}}{\textcolor{red}{\text{RFM}}}$$

calculating the percentage composition of sodium in sodium sulphate ($Na_2SO_4$):

Step-by-step example: $Na_2SO_4$

Let's find the percentage by mass of sodium (Na) in sodium sulphate (Na_2SO_4).

1. Find the Relative Atomic Masses (Ar):

• Sodium (Na): 23
• Sulphur (S): 32
• Oxygen (O): 16

2. Calculate the Relative Formula Mass (RFM) of $Na_2SO_4$.

$$\text{RFM} = (23 \times 2) + 32 + (16 \times 4)$$ $$\text{RFM} = 46 + 32 + 64 = 142$$

3. Calculate the Mass of Sodium in $Na_2SO_4$:

• Sodium contributes: $23 \times 2 = 46$

4. Find the % Mass of Sodium:

$$\text{\% mass of Na} = \frac{46}{142} \times 100$$ $$\text{\% mass of Na} = 32.4\%$$