Calculating Moles from Mass

Converting mass to moles is the single most-used calculation step in IGCSE Chemistry. It is step 2 of the universal mole method: once you have moles, you can use the mole ratio to find anything else.

The method

1. Write down the mass given in the question. Convert to grams if necessary.
2. Calculate or look up the Mr of the substance.
3. Divide the mass by Mr to get the number of moles.

The formula

n = mass / Mr

Where:

• n = number of moles (mol)
• mass = mass of the substance in grams (g)
• Mr = relative formula mass (no units, but used as g/mol in calculations)

Rearranged: mass = n x Mr, and Mr = mass / n.

Worked examples

Example 1: Moles of water

Question: Calculate the number of moles in 36 g of water, H2O. (Ar: H = 1, O = 16)

Working:

• Mr of H2O = (2 x 1) + 16 = 18
• n = mass / Mr = 36 / 18 = 2.0 mol

Answer: 2.0 mol

Examiner note: Always show the Mr calculation. It earns its own mark.

Example 2: Moles of sodium chloride

Question: Calculate the number of moles in 11.7 g of sodium chloride, NaCl. (Ar: Na = 23, Cl = 35.5)

Working:

• Mr of NaCl = 23 + 35.5 = 58.5
• n = mass / Mr = 11.7 / 58.5 = 0.200 mol

Answer: 0.200 mol

Examiner note: Chlorine has Ar = 35.5 on the IGCSE periodic table, not 35. Using the wrong value gives a wrong answer even though the method is correct.

Example 3: Moles of calcium carbonate

Question: Calculate the number of moles in 5.00 g of calcium carbonate, CaCO3. (Ar: Ca = 40, C = 12, O = 16)

Working:

• Mr of CaCO3 = 40 + 12 + (3 x 16) = 100
• n = mass / Mr = 5.00 / 100 = 0.0500 mol

Answer: 0.0500 mol

Examiner note: This is one of the most common setups in Paper 4. CaCO3 appears repeatedly because its Mr of 100 makes the arithmetic straightforward so the examiner can test method, not calculator skills.

Example 4: Moles of magnesium (element)

Question: Calculate the number of moles in 4.8 g of magnesium. (Ar: Mg = 24)

Working:

• Ar of Mg = 24 (for an element, Ar takes the place of Mr)
• n = mass / Ar = 4.8 / 24 = 0.20 mol

Answer: 0.20 mol

Examiner note: Magnesium is a monatomic metal. Use the Ar directly. Do not double it.

Example 5: Moles of oxygen gas (diatomic)

Question: Calculate the number of moles in 6.4 g of oxygen gas, O2. (Ar: O = 16)

Working:

• Mr of O2 = 2 x 16 = 32
• n = mass / Mr = 6.4 / 32 = 0.20 mol

Answer: 0.20 mol

Examiner note: Oxygen gas is O2, not O. Using Ar = 16 instead of Mr = 32 gives double the correct answer. Always check whether the question says “oxygen gas” or “oxygen atoms.”

Example 6: Mass given in kilograms

Question: A reaction produces 0.250 kg of iron. Calculate the number of moles of iron produced. (Ar: Fe = 56)

Working:

• Convert to grams: 0.250 kg = 250 g
• Ar of Fe = 56
• n = mass / Ar = 250 / 56 = 4.46 mol

Answer: 4.46 mol (3 significant figures)

Examiner note: The unit trap is deliberate. If you use 0.250 directly without converting to grams, your answer is 1000 times too small. Check units before substituting.

Common mistakes

1. Using Ar instead of Mr for a molecule. O2 needs Mr = 32, not Ar = 16. Always check the formula.
2. Not converting kilograms to grams. 1 kg = 1000 g. The formula only works with grams.
3. Dividing the wrong way. It is mass divided by Mr, not Mr divided by mass.
4. Skipping the Mr calculation. Even if Mr is given, examiners may award a separate mark for showing you understand where it came from.
5. Rounding prematurely. Keep at least 3 significant figures during intermediate steps and round only the final answer.

When this appears

This calculation is the entry point to nearly all stoichiometry questions. Once you have moles, you can find reacting masses, gas volumes, or concentrations. The reverse calculation, mass from moles, uses the same formula rearranged. The stoichiometry topic covers the theory of the mole, Avogadro’s constant, and why this conversion works.