Calculate Relative Formula Mass (AQA GCSE Chemistry): Revision Note

Calculate the relative formula mass of:

1. Sodium chloride, NaCl

2. Copper oxide, CuO

3. Magnesium nitrate, Mg(NO₃)₂

Answers:

1. Sodium chloride

   • NaCl = 23 + 35.5 = 58.5

2. Copper oxide

   • CuO = 63.5 + 16 = 79.5

3. Magnesium nitrate

   • Mg(NO₃)₂ = 24 + (14 x 1 x 2) + (16 x 3 x 2) = 148

The relative atomic mass of every element is given on the Periodic Table. It is the larger of the two numbers.

• The most common incorrect answer for percentage composition are values above 100%

• This is not possible and usually happens when the numbers in the calculation are "upside down"

  • For example, the percentage composition of hydrogen in water:

    • CORRECT = $\left(\frac{2\times 1}{18}\right)$ x 100 = 11.1%

    • INCORRECT = $\left(\frac{18}{2\times 1}\right)$ x 100 = 900%

Calculate the percentage of carbon in the formula of glucose, C₆H₁₂O₆.

Answer:

• From the Periodic Table, the relative atomic masses are:

  • Carbon = 12

  • Hydrogen = 1

  • Oxygen = 16 

• The total mass of glucose is:

  • (6 × 12) + (12 × 1) + (6 × 16) = 180

• The equation for percentage composition is:

  • Percentage composition = $\left(\frac{\mathrm{total}\mathrm{mass}\mathrm{of}\mathrm{element}}{\mathrm{total}\mathrm{mass}\mathrm{of}\mathrm{compound}}\right)$ × 100

• So, the percentage composition of carbon in glucose is:

  • Percentage of carbon = $\left(\frac{6\times 12}{180}\right)$ × 100 = 40%

The chemical formula of the fertiliser ammonium nitrate is NH₄NO₃. Calculate the percentage by mass of nitrogen in ammonium nitrate.

Answer:

• From the Periodic Table, the relative atomic masses are:

  • Nitrogen = 14

  • Hydrogen = 1

  • Oxygen = 16 

• The total mass of ammonium nitrate is:

  • (1 x 14) + (4 × 1) + (1 x 14) + (3 × 16) = 80

• The equation for percentage composition is:

  • Percentage Composition = $\left(\frac{\mathrm{total}\mathrm{mass}\mathrm{of}\mathrm{element}}{\mathrm{total}\mathrm{mass}\mathrm{of}\mathrm{compound}}\right)$ × 100

• So, the percentage composition of nitrogen in ammonium nitrate is:

  • Careful: There are two nitrogen atoms in ammonium nitrate

  • Percentage of nitrogen = $\left(\frac{2\times 14}{80}\right)$ × 100 = 35%