Magnesium nitrate has the formula Mg(NO₃)₂. State what is meant by the term relative formula mass and describe how the relative formula mass of magnesium nitrate is determined from its formula.

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

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

Magnesium nitrate, Mg(NO₃)₂, is used as a fertiliser. The relative atomic masses are: Mg = 24, N = 14, O = 16.

Model answer (4 marks)

Relative formula mass is the sum of the relative atomic masses of all the atoms in the formula.

For Mg(NO₃)₂ there is 1 Mg, 2 N and 6 O.

So the relative formula mass = 24 + (2×14) + (6×16) = 24 + 28 + 96 = 148.

Examiner tips

  • Use the exact definition of relative formula mass. Show the count of each atom from the formula. Multiply each atomic mass by its count. Give the final numerical value with no units.

Common mistakes

  • Missing the factor of 2 for the two nitrate groups. Using incorrect atomic masses or forgetting to multiply. Giving the answer as a mass in grams instead of a relative formula mass.

Mark scheme (4 marks)

  1. Relative formula mass is the sum of the relative atomic masses of all the atoms in the formula
  2. Correctly identifies the number of each type of atom: 1 Mg, 2 N, 6 O
  3. Correctly multiplies each relative atomic mass by the number of those atoms: 24 + (2 × 14) + (6 × 16)
  4. Arrives at the correct relative formula mass of 148

Key terms in this question

relative formula mass · formula

Related

More Relative formula mass questions

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