The unit of amount of substance; one mole contains 6.02 × 10²³ particles.

What is the Avogadro constant?

L = 6.02 × 10²³ mol⁻¹, the number of particles in one mole.

The mass per mole of a substance, in g mol⁻¹.

Define empirical formula.

The simplest whole-number ratio of atoms of each element in a compound.

Define molecular formula.

The actual number of atoms of each element in one molecule.

How do you find an empirical formula from percentages?

Divide each percentage by Ar, divide by the smallest, then convert to whole numbers.

How do you find the molecular formula?

Divide M by the empirical formula mass and multiply the empirical formula by the result.

SI units to use in pV = nRT.

p in Pa, V in m³, T in K, R = 8.31 J K⁻¹ mol⁻¹.

Convert 250 cm³ to m³.

2.50 × 10⁻⁴ m³ (1 cm³ = 10⁻⁶ m³).

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Define the mole.: The unit of amount of substance; one mole contains 6.02 × 10²³ particles.
What is the Avogadro constant?: L = 6.02 × 10²³ mol⁻¹, the number of particles in one mole.
What is molar mass?: The mass per mole of a substance, in g mol⁻¹.
Formula linking amount, mass and molar mass.: n = m ÷ M
Formula linking number of particles to amount.: N = n × L
Define empirical formula.: The simplest whole-number ratio of atoms of each element in a compound.
Define molecular formula.: The actual number of atoms of each element in one molecule.
How do you find an empirical formula from percentages?: Divide each percentage by Ar, divide by the smallest, then convert to whole numbers.
What do you do with a ratio such as 1 : 1.5?: Multiply by 2 to get 2 : 3.
How do you find the molecular formula?: Divide M by the empirical formula mass and multiply the empirical formula by the result.
In combustion analysis, how do you find n(H) from water?: n(H) = 2 × n(H₂O).
Write the ideal gas equation.: pV = nRT
SI units to use in pV = nRT.: p in Pa, V in m³, T in K, R = 8.31 J K⁻¹ mol⁻¹.
Convert 250 cm³ to m³.: 2.50 × 10⁻⁴ m³ (1 cm³ = 10⁻⁶ m³).
Expression for molar mass of a volatile liquid from vapour data.: M = mRT ÷ pV

Exam questions on The mole and molar mass

A technician uses a 22.0 g block of dry ice (solid carbon dioxide, CO₂) for a school demonstration. Relative atomic masses: C 12.0, O 16.0. The Avogadro constant, L, is 6.02 × 10²³ mol⁻¹ and the gas constant, R, is 8.31 J K⁻¹ mol⁻¹.
The block turns completely into gas in a large bag at 298 K and 101 kPa. Calculate the volume of the gas in dm³, assuming it behaves as an ideal gas.2 marks
An organic liquid contains only carbon, hydrogen and oxygen. Analysis shows that it contains 54.5% carbon, 9.1% hydrogen and 36.4% oxygen by mass. Relative atomic masses: H 1.0, C 12.0, O 16.0. The gas constant, R, is 8.31 J K⁻¹ mol⁻¹.
A 0.320 g sample of the liquid is vaporised and occupies 112 cm³ at 373 K and 101 kPa. Calculate its molar mass and deduce its molecular formula.2 marks
A student heats 2.46 g of hydrated magnesium sulfate, MgSO₄·xH₂O, until it reaches constant mass. The anhydrous magnesium sulfate left has a mass of 1.20 g. M(MgSO₄) = 120.4 g mol⁻¹ and M(H₂O) = 18.0 g mol⁻¹. The Avogadro constant, L, is 6.02 × 10²³ mol⁻¹.
Calculate the amount (mol) of anhydrous magnesium sulfate and of water driven off, and hence find the value of x.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).