What are the units of relative atomic mass?

None. It is a ratio, so the units cancel.

Why is the relative atomic mass of chlorine 35.5?

It is a weighted mean of its isotopes (35Cl and 37Cl) allowing for their abundances.

An atom or group of atoms with an overall charge because it has gained or lost electrons.

Two or more atoms joined by covalent bonds.

A substance with two or more elements chemically bonded in a fixed ratio.

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.

When is 'relative formula mass' used instead of relative molecular mass?

For compounds with giant structures, such as ionic lattices, which have no discrete molecules.

What are the units of molar mass?

g mol-1, the mass per mole of a substance.

Atoms, ions, formulae and relative massesEdexcel International 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 relative to one twelfth of the mass of an atom of carbon-12.
What are the units of relative atomic mass?: None. It is a ratio, so the units cancel.
Why is the relative atomic mass of chlorine 35.5?: It is a weighted mean of its isotopes (35Cl and 37Cl) allowing for their abundances.
What is an ion?: An atom or group of atoms with an overall charge because it has gained or lost electrons.
What is a molecule?: Two or more atoms joined by covalent bonds.
What is a compound?: A substance with two or more elements chemically bonded in a fixed ratio.
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.
When is 'relative formula mass' used instead of relative molecular mass?: For compounds with giant structures, such as ionic lattices, which have no discrete molecules.
What are the units of molar mass?: g mol-1, the mass per mole of a substance.
Work out the relative formula mass of Ca(OH)2 (Ca 40.1, O 16.0, H 1.0).: 40.1 + 2 x (16.0 + 1.0) = 74.1
What does 1 ppm mean?: One part in 10^6 of the total.
Convert 0.050% to ppm.: 0.050 x 10 000 = 500 ppm

Exam questions on Atoms, ions, formulae and relative masses

A garden-centre chemist checks the label on a fertiliser that contains ammonium sulfate, (NH₄)₂SO₄. Relative atomic masses: H = 1.0, N = 14.0, O = 16.0, S = 32.1.
Calculate the percentage by mass of nitrogen in ammonium sulfate. Give your answer to 3 significant figures.2 marks
Chemists measure relative atomic masses on the carbon-12 scale. A data book gives the relative atomic mass of magnesium as 24.3, although naturally occurring magnesium contains only atoms with mass numbers 24, 25 and 26.
Explain why the relative atomic mass of magnesium has no units, whereas the molar mass of magnesium is 24.3 g mol⁻¹.2 marks
A school air-quality sensor reports the carbon dioxide concentration in a sealed classroom of volume 180 m³ as 1250 ppm by volume. Dry air contains 0.93% argon by volume. 1 m³ = 1000 dm³.
Calculate the volume, in dm³, of carbon dioxide in the classroom.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).