Molar mass (M) is the mass of one mole of a substance, in grams per mole. It is numerically equal to the relative atomic or molecular mass but carries a unit, and you can find it from M = m / n to identify an unknown substance.
Molar mass is the bridge between the mass you can weigh and the moles you calculate with, so it sits at the centre of the mole chapter in SPM Chemistry. Our online one-to-one teachers make sure you never confuse it with relative molecular mass, because the difference, a unit, is exactly what a careful marking scheme checks.
When you use it
You use molar mass in every mole-and-mass conversion, and again in a special SPM favourite: identifying an unknown element or compound from a measured mass and a known number of moles. Recognising when a question is really asking “what is M?” is half the battle.
The definition and the formula
The molar mass of a substance is the mass of one mole of it. Numerically it equals the relative atomic mass (element) or relative molecular / formula mass (compound), but it carries the unit grams per mole (g mol⁻¹).
To find it from data, rearrange the mole formula:
M = m / n
- M = molar mass, in g mol⁻¹
- m = mass, in grams (g)
- n = number of moles, in mol
Units
Molar mass is always in g mol⁻¹. This unit is what distinguishes it from the relative molecular mass, which is a pure ratio with no unit.
Worked example 1 (easy), oxygen gas
State the molar mass of oxygen gas, O2. (Ar: O = 16.)
- Step 1, oxygen gas is diatomic: 2 x 16 = 32.
- Step 2, attach the unit: M(O2) = 32 g mol⁻¹.
Note that the molar mass of oxygen atoms, O, is 16 g mol⁻¹, so always check whether the question means the atom or the molecule.
Worked example 2 (medium), molar mass from data
0.25 mol of a compound has a mass of 20 g. Find its molar mass.
- Step 1, M = m / n = 20 / 0.25.
- Step 2, M = 80 g mol⁻¹.
Worked example 3 (SPM level), identifying a metal
0.1 mol of a Group 2 metal has a mass of 4.0 g. Identify the metal.
- Step 1, M = m / n = 4.0 / 0.1 = 40 g mol⁻¹.
- Step 2, a Group 2 metal with molar mass 40 g mol⁻¹ is calcium (Ar of Ca = 40).
So the metal is calcium. This “find M, then match it to an element” method is a very common SPM structured-question step.
Common traps
- Confusing molar mass (g mol⁻¹) with relative molecular mass (no unit). Write the unit whenever the question asks for molar mass.
- Using the atomic value when the substance is a molecule, or vice versa (O vs O2, H vs H2).
- Turning M = m / n upside down.
- Forgetting to convert kilograms to grams before dividing.
- Rounding a metal’s molar mass so it no longer matches a real element.
How we help
Our teachers give you both directions, building M from a formula, and finding M from data to name an unknown, so you recognise instantly which the question wants. Lessons are one-to-one, taught in English, from RM50 per hour, with a paid one-hour trial so you can try it before committing. Because molar mass links mass, moles and identity, getting it firmly right makes reacting-mass and empirical-formula questions much easier later in the chapter and in Paper 2.
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