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ChemistryGCSE, IGCSE & A Level

What is mole?

A mole is the amount of substance containing 6.02 × 10²³ particles — the chemist's counting unit that links mass to number of particles.

Mole explained

Moles convert between grams, particles, gas volumes and solution concentrations, which is why almost every quantitative chemistry question starts by calculating them.
Balanced equations give mole ratios, so once you have the moles of one substance you can find the moles, and then the mass, of anything else in the reaction.

Key formula

n=mMr,n=cVn = \dfrac{m}{M_{r}}, \qquad n = cV

Worked example

44 g44\,\text{g} of CO2\mathrm{CO_{2}} (Mr=44M_{r} = 44) is exactly 11 mole, containing 6.02×10236.02 \times 10^{23} molecules.

Examiner tip

Lay calculations out in three steps — moles, ratio, answer. Examiners award method marks for each step even when the final number is wrong.

Mole: common questions

What is Avogadro's constant?
6.02 × 10²³ mol⁻¹ — the number of particles in one mole of any substance.
How do I find moles of a gas?
At room temperature and pressure, divide the volume in dm³ by 24; otherwise use the ideal gas equation pV = nRT.

Related terms

Molar mass

Molar mass is the mass of one mole of a substance in grams, numerically equal to its relative formula mass.

Concentration

Concentration is the amount of solute dissolved in a given volume of solution, usually in moles per cubic decimetre.

Limiting reagent

The limiting reagent is the reactant that is fully used up first and therefore determines the maximum yield of product.

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