AQA Chemistry 5.3.2.1 · Higher only

Moles

A mole is a count: 6.02 × 10²³ particles. Its mass in grams is the relative formula mass. Two equations, each taught on its own, then chained.

Combined Science · Higher tier

Calculates

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Two dozens.

One bag holds a dozen eggs. Another holds a dozen melons. Same number in each.

If you had to guess, which bag is heavier?

The mole

One number, any substance.

Chemical amounts are measured in moles (mol). The mass of one mole of a substance in grams is numerically equal to its relative formula mass. One mole of any substance contains the same number of particles: 6.02 × 10²³, the Avogadro constant, N_A. The particles can be atoms, molecules, ions, electrons or formula units. Say which you are counting: one mole of water is 6.02 × 10²³ molecules, and each molecule has three atoms, so it is 3 × 6.02 × 10²³ atoms.

Standard form, before you need it

6.02 × 10²³ means 6.02 with the decimal point moved 23 places to the right: 602 followed by 21 zeros. To multiply by a number, multiply the front part: 2 × 6.02 × 10²³ = 12.04 × 10²³ = 1.204 × 10²⁴, because in standard form the front number must be between 1 and 10. To divide two numbers with the same power of ten, divide the front parts: 3.01 × 10²³ ÷ 6.02 × 10²³ = 0.5.

What is 0.1 × 6.02 × 10²³ in standard form?

Pick one.

Equation 1 · moles from mass

Mass in grams, always. M_r has no unit (use A_r for an element). Tap the quantity you want to find.

Equation

Moles from mass

Side by side: multiply.
One over the other: divide.

Tap the quantity you want to find.

Written out

n = m ÷ Mr

m: mass; g
n: amount of substance; mol
Mr: relative formula mass (Ar for an element); no unit

Equation 2 · particles from moles

N_A is a fixed number, 6.02 × 10²³ per mole, so it cannot be the thing you find. Tap the other two.

Equation

Particles from moles

Side by side: multiply.
One over the other: divide.

Tap the quantity you want to find.

Written out

particles = n × Nₐ

N: number of particles; no unit
n: amount of substance; mol
Nₐ: Avogadro constant, 6.02 × 10²³; per mol

Think again · same number, same mass?

“A mole of anything weighs the same.”

Your call

Choose

One mole of water (Mr 18) and one mole of carbon dioxide (Mr 44). What do they have in common?

Sort

Which line first?

For each question, which is the first thing you write?

Tap a card, then tap the box it belongs in. Tap a placed card to take it back.

Also on the specification, taught elsewhere

Percentage by mass of an element in a compound is Foundation content and lives in Relative formula mass. Using moles to balance equations and to find a limiting reactant is in Using moles in calculations. Reacting masses are next: Amounts of substances in equations.

Key fact

n = m ÷ M_r, with m in grams. Particles = n × 6.02 × 10²³. One mole of any substance holds the same number of particles; its mass in grams is its M_r.

Key note · AQA 5.3.2.1 · Higher only

Moles

Chemical amounts are measured in moles (mol). The mass of one mole of a substance in grams is numerically equal to its relative formula mass.
One mole of a substance contains the same number of particles, atoms, molecules or ions as one mole of any other substance: the Avogadro constant, 6.02 × 10²³ per mole.
n = m ÷ Mr, with the mass in grams: convert kg to g first (2.7 kg Al = 2700 g = 100 mol). Mr has no unit.
particles = n × Nₐ; n = particles ÷ Nₐ. Standard form: keep the front number between 1 and 10 (12.04 × 10²³ = 1.204 × 10²⁴).
Chained: mass from a count is particles → moles → mass; a count from a mass is mass → moles → particles. Percentage by mass is in Relative formula mass.

Photograph this card. Tomorrow, cover it and say each line first.

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Putting kilograms straight into n = m ÷ Mr?

Relative atomic masses are the AQA periodic-table values rounded as given in each question.