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Relative formula mass – practice for Years 8–9

Sample questions on Relative formula mass with the right answer and a worked solution, from easy to hard. In the Pickles Academy app a child gets new questions of the same kind every time, so they can't just learn the answers by heart.

Subject
Chemistry
Year
Years 8–9
Curriculum area
Atoms and elements

Question types: Work out Mr

Rule: Relative formula mass

Mr is the sum of the relative atomic masses of all the atoms in the formula. The small number after a symbol says how many atoms.

Brackets multiply everything inside: Ca(OH)₂ = Ca + 2 × O + 2 × H.

Example: Al₂O₃: 2 × 27 + 3 × 16 = 54 + 48 = 102

Watch out: The small number belongs only to the symbol before it: H₂O has 2 H atoms but only 1 O atom.

Sample questions

  1. Question 1 Easy

    Work out the relative formula mass Mr of NH₃. Relative atomic masses: N = 14, H = 1.

    Show the solution
    Answer: 17

    How it works: Mr(NH₃) = 14 + 3 × 1 = 17.

  2. Question 2 Normal

    Work out the relative formula mass Mr of Al₂O₃. Relative atomic masses: Al = 27, O = 16.

    Show the solution
    Answer: 102

    How it works: Mr(Al₂O₃) = 2 × 27 + 3 × 16 = 102.

  3. Question 3 Hard

    Work out the relative formula mass Mr of Mg(OH)₂. Relative atomic masses: Mg = 24, O = 16, H = 1.

    Show the solution
    Answer: 58

    How it works: Mr(Mg(OH)₂) = 24 + 2 × 16 + 2 × 1 = 58.

Related topics

All Chemistry topics

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