Explain what is meant by the term 'molar mass' and outline how the molar mass of a substance can be determined from the periodic table, using magnesium chloride, MgCl₂, as your example.

IB DP Chemistry Standard Level (2023 syllabus) — S1.4 Counting particles by mass: The mole · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Molar mass is the mass of one mole of a substance.
It is expressed in units of g mol⁻¹.
The molar mass of an element is numerically equal to its relative atomic mass on the periodic table.
For MgCl₂ the molar mass is found by adding the relative atomic masses: Mg (24.31) + 2 × Cl (35.45) = 95.2 g mol⁻¹ (approx).

Examiner tips

  • Define molar mass and give its units; link to Avogadro’s number. Show the calculation step‑by‑step using the periodic table values. Mention that the result is an approximate value. Use correct symbols (MgCl₂) and units (g mol⁻¹).

Common mistakes

  • Confusing molar mass with molar volume or molar concentration. Using incorrect atomic masses or forgetting to multiply by 2 for Cl. Omitting the units g mol⁻¹.

Mark scheme (4 marks)

  1. Molar mass is the mass of one mole of a substance
  2. Molar mass has units of g mol⁻¹
  3. The molar mass of an element is numerically equal to its relative atomic mass found on the periodic table
  4. For MgCl₂, the molar mass is calculated by summing the relative atomic masses: Mg (24.31) + 2 × Cl (35.45) = approximately 95.2 g mol⁻¹

Key terms in this question

molar mass

Related

More Counting particles by mass: The mole questions

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