Define relative molecular mass.

The weighted mean mass of a molecule compared with 1/12 of the mass of an atom of ¹²C.

When is the term relative formula mass used?

For ionic compounds, which do not contain molecules.

What is the Avogadro constant?

The number of particles in one mole, 6.02 × 10²³ mol⁻¹.

What is the relationship between the Avogadro constant and the mole?

One mole of a substance contains 6.02 × 10²³ particles, so number of particles = amount × 6.02 × 10²³.

How many hydrogen atoms are in 0.100 mol of H₂O?

0.200 mol × 6.02 × 10²³ = 1.20 × 10²³ atoms.

Calculate the Mr of Ca(OH)₂.

40.1 + 2 × (16.0 + 1.0) = 74.1

Relative masses and the moleAQA A-Level Chemistry: Flashcards

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Define relative atomic mass.

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Define relative atomic mass.: The weighted mean mass of an atom of an element compared with 1/12 of the mass of an atom of ¹²C.
Define relative molecular mass.: The weighted mean mass of a molecule compared with 1/12 of the mass of an atom of ¹²C.
When is the term relative formula mass used?: For ionic compounds, which do not contain molecules.
What units do relative atomic and relative molecular mass have?: None, because they are ratios.
What is the Avogadro constant?: The number of particles in one mole, 6.02 × 10²³ mol⁻¹.
What is the relationship between the Avogadro constant and the mole?: One mole of a substance contains 6.02 × 10²³ particles, so number of particles = amount × 6.02 × 10²³.
How do you calculate amount in moles from mass?: Amount = mass (g) ÷ Mr.
What is the formula for the amount of substance in a solution?: n = c × V, with V in dm³.
How do you convert cm³ to dm³?: Divide by 1000.
What is the unit of concentration in this course?: mol dm⁻³.
How many hydrogen atoms are in 0.100 mol of H₂O?: 0.200 mol × 6.02 × 10²³ = 1.20 × 10²³ atoms.
Calculate the Mr of Ca(OH)₂.: 40.1 + 2 × (16.0 + 1.0) = 74.1

Exam questions on Relative masses and the mole

Magnesium nitrate, Mg(NO₃)₂, is used in some fireworks as an oxidising agent. A technician needs to convert between masses and amounts of this compound, using relative atomic masses Mg 24.3, N 14.0 and O 16.0.
Calculate the amount, in mol, of magnesium nitrate in 7.42 g of Mg(NO₃)₂. Give your answer to 3 significant figures.2 marks
A student has 3.60 g of water, H₂O, in a beaker. Use relative atomic masses H 1.0 and O 16.0, and the Avogadro constant, 6.02 × 10²³ mol⁻¹.
Calculate the mass, in grams, of one molecule of water. Give your answer in standard form to 3 significant figures.2 marks
A technician prepares a solution of sodium carbonate, Na₂CO₃, by dissolving 6.36 g of the solid in water and making the volume up to exactly 250 cm³ in a volumetric flask. Use relative atomic masses Na 23.0, C 12.0 and O 16.0, and the Avogadro constant, 6.02 × 10²³ mol⁻¹.
Calculate the concentration of the sodium carbonate solution in mol dm⁻³.3 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).