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K324 / 2027
Chemical Calculations

Lesson 3 of 6

The mole is a counting unit

Connect microscopic particles to measurable mass.

O-Level 6092 (2026) / SEC G3 K324 (2027)

  • I can define relative atomic mass.Syllabus

    Syllabus K324, 4.2(a). Define relative atomic mass

  • I can define and calculate relative molecular/formula mass.Syllabus

    Syllabus K324, 4.2(b). Define and calculate relative molecular/formula mass

  • I can define the mole using the Avogadro constant.Syllabus

    Syllabus K324, 4.2(c). Define the mole using the Avogadro constant

The mole is a counting unit

Connect microscopic particles to measurable mass.

Make a guess. You are not marked.

You weigh out 12 g of carbon and 12 g of magnesium. Use C = 12 and Mg = 24. Which sample contains more atoms?

  1. Both contain the same number of atoms
  2. The 12 g of magnesium, as its atoms are bigger
  3. The 12 g of carbon, as its atoms are lighter
Show the answer

The 12 g of carbon, as its atoms are lighter

Carbon atoms are half as heavy, so 12 g holds 1 mol of carbon atoms but only 0.5 mol of magnesium atoms.

Relative atomic mass, Ar, compares the average mass of an atom with one twelfth of the mass of a carbon-12 atom. Relative molecular mass, Mr, makes the same comparison for a molecule. Add the Ar values in its formula. For an ionic compound, call this relative formula mass instead. These relative masses have no units.

One mole contains the Avogadro constant number of specified entities: approximately 6.02 x 1023. Specify atoms, molecules, ions or formula units. The molar mass M has units g mol-1 and is numerically equal to the relative molecular or formula mass. Use n = m/M and particle count = n x NA.

Worked exampleAccount for brackets in a formula

Find the relative formula mass of Ca(OH)2 and the amount in 3.70 g. Use Ca = 40, O = 16, H = 1.

Try it first. Then show the worked stepsHide the worked steps
  1. Mr = 40 + 2(16 + 1) = 74.
  2. Molar mass = 74 g mol-1.
  3. n = 3.70/74 = 0.0500 mol.
Answer

0.0500 mol of Ca(OH)2 formula units; if fully dissolved, this produces 0.100 mol of OH- ions.

Try it step by step

Work it out in steps: Molar mass, then Amount in moles.

  1. Problem 1. Find the amount of sodium carbonate, Na2CO3, in 5.30 g. Use Na = 23, C = 12, O = 16. Give the amount in mol to 3 significant figures.
  2. Problem 2. Find the amount of magnesium sulfate, MgSO4, in 6.00 g. Use Mg = 24, S = 32, O = 16. Give the amount in mol to 3 significant figures.
  3. Problem 3. Find the amount of calcium nitrate, Ca(NO3)2, in 4.10 g. Use Ca = 40, N = 14, O = 16. Give the molar mass in g mol-1 and the amount in mol to 3 significant figures.
Show the answer

Problem 1.

Molar mass: M(Na2CO3) = 2(23) + 12 + 3(16) = 106 g mol-1. Count every atom in the formula: two Na, one C and three O.

Amount in moles: n = m / M = 5.30 / 106 = 0.0500 mol of Na2CO3 formula units.

Problem 2.

Molar mass: M(MgSO4) = 24 + 32 + 4(16) = 120 g mol-1.

Amount in moles: n = m / M = 6.00 / 120 = 0.0500 mol. Write 0.0500, not 0.05, to show 3 significant figures.

Problem 3.

Molar mass: M(Ca(NO3)2) = 40 + 2(14 + 3 x 16) = 40 + 2(62) = 164 g mol-1. The 2 outside the bracket doubles everything inside it.

Amount in moles: n = m / M = 4.10 / 164 = 0.0250 mol of Ca(NO3)2.

Check your understanding

How many moles of oxygen atoms are in 0.25 mol of O2 molecules?

  1. 0.50 mol of oxygen atoms, since each molecule has two
  2. 0.125 mol of oxygen atoms, half the molecules
  3. 0.25 mol of oxygen atoms, the same as the molecules
Show the answer

0.50 mol of oxygen atoms, since each molecule has two

Each O2 molecule holds two O atoms, so 0.25 x 2 = 0.50 mol of atoms. The amount of molecules is still 0.25 mol; always say which particle you are counting.