Molar Mass – IGCSE Chemistry Definition

Molar mass is the mass of one mole of a substance. It is the bridge between the unitless relative molecular mass on the periodic table and the grams you weigh in the lab. Every mass-to-moles conversion on the 0620 syllabus uses molar mass.

The 0620 definition

The molar mass (M) of a substance is the mass of one mole of that substance, in grams per mole (g/mol). It is numerically equal to the relative formula mass (Mr) or relative atomic mass (Ar).

The key relationship

moles = mass (g) / molar mass (g/mol)

Rearranged:

mass = moles x molar mass

molar mass = mass / moles

Examples

Substance	Mr	Molar mass	Meaning
Carbon, C	12	12 g/mol	1 mol of carbon atoms weighs 12 g
Water, H₂O	18	18 g/mol	1 mol of water molecules weighs 18 g
NaOH	40	40 g/mol	1 mol of NaOH weighs 40 g
H₂SO₄	98	98 g/mol	1 mol of sulfuric acid weighs 98 g
CaCO₃	100	100 g/mol	1 mol of calcium carbonate weighs 100 g

Using molar mass in calculations

Example 1: mass to moles

How many moles in 5.6 g of iron? (Molar mass of Fe = 56 g/mol)

moles = 5.6 / 56 = 0.1 mol

Example 2: moles to mass

What is the mass of 0.25 mol of CaCO₃? (Molar mass = 100 g/mol)

mass = 0.25 x 100 = 25 g

Worked exam question

Calculate the mass of 0.4 mol of copper(II) sulfate, CuSO₄. (Ar: Cu = 64, S = 32, O = 16) (2)

Mark scheme

Molar mass = 64 + 32 + 4(16) = 160 g/mol [1]; mass = 0.4 x 160 = 64 g [1]

Common exam mistakes

• Confusing molar mass with mass. Molar mass is the mass of one mole, not the mass of any arbitrary amount.
• Using Ar instead of Mr for compounds. The molar mass of NaCl is 23 + 35.5 = 58.5 g/mol, not 23 g/mol or 35.5 g/mol.
• Forgetting the unit. Relative atomic mass and Mr have no units, but molar mass has the unit g/mol. In calculations this distinction matters for showing your working.