Aluminium chloride has the formula AlCl₃. State what is meant by the term relative formula mass and describe the steps you would follow to find the relative formula mass of aluminium chloride. Use the relative atomic masses: Al = 27, Cl = 35.5.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.4 Relative formula mass · State and Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Relative formula mass is the sum of the relative atomic masses of all the atoms in a formula.
1. Identify the atoms in AlCl₃ – one Al atom and three Cl atoms.
2. Multiply each atomic mass by its number of atoms:
1 × 27 = 27
3 × 35.5 = 106.5
3. Add the results: 27 + 106.5 = 133.5
Therefore the relative formula mass of aluminium chloride is 133.5.

Examiner tips

  • State the definition first, then list the steps in order.
  • Show the multiplication for each element before adding.
  • Use the given atomic masses exactly as provided.

Common mistakes

  • Using the wrong atomic mass for Cl (e.g., 35 instead of 35.5).
  • Omitting the multiplication step and just adding the masses.
  • Writing the final mass as 134 instead of 133.5.

Mark scheme (4 marks)

  1. Relative formula mass is the sum of the relative atomic masses of all the atoms in a formula
  2. Identify the number of atoms of each element: one Al atom and three Cl atoms
  3. Multiply the relative atomic mass of each element by the number of its atoms: 1 × 27 = 27 and 3 × 35.5 = 106.5
  4. Add the results to obtain the relative formula mass: 27 + 106.5 = 133.5

Key terms in this question

relative formula mass · relative atomic mass · formula

Related

More Relative formula mass questions

▶ Try answering this question with AI marking (free) →