AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Relative atomic mass and relative molecular mass in terms of C.

    Full specification wording

    Relative atomic mass and relative molecular mass: Relative atomic mass and relative molecular mass in terms of C.

    Relative atomic mass is a ratio, so it has no units.

    Checked against the AQA 7405 specification (opens in a new tab) on .

    • Point 1 of 2
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Relative atomic mass is a ratio, so it has no units. It is the weighted mean mass of an atom of the element divided by one-twelfth the mass of an atom of carbon-12. Both halves of that definition carry a mark, and the two halves must be consistent: atoms compared with atoms, or moles compared with moles, never one of each. Carbon-12 is the reference because its mass is defined as exactly 12 on this scale. Relative molecular mass is the same idea applied to a molecule: the mean mass of one molecule of the substance divided by one-twelfth the mass of one atom of carbon-12, obtained in practice by adding the relative atomic masses of every atom in the formula.

    What earns the marks

    • The numerator given as the average or mean mass of an atom of the element.
    • The denominator given as one-twelfth the mass of one atom of carbon-12.
    • Relative molecular mass defined as the mean mass of a molecule on the same scale.
    • Keeping atoms with atoms or moles with moles throughout the definition.

    Common mistakes and how to put them right

    • Mixing moles in one half of the definition with atoms in the other, which is capped at one mark.

    • Writing mass of one atom without the word average or mean.

    • Comparing with the mass of a carbon-12 atom while forgetting the one-twelfth.

    • Giving units such as grams to a quantity that is a ratio.

    Exam tips

    • Write the definition as a fraction with a horizontal line so the examiner can mark each line separately.
    • Say carbon-12 explicitly rather than just carbon; ordinary carbon is a mixture of isotopes.
    • Add average or weighted mean to the top line even when the question seems to be about a single atom.

    Try the lesson

    The start of the Relative atomic mass and relative molecular mass: Relative atomic mass and relative molecular mass in terms of C. lesson, made from checked teaching notes for this specification point.

    Have a go first

    How is the relative molecular mass of a substance defined?

    Listen

    Relative atomic mass is a ratio, so it has no units. It is the weighted mean mass of an atom of the element divided by one-twelfth the mass of an atom of carbon-12. Both halves of that definition carry a mark, and the two halves must be consistent: atoms compared with atoms, or moles compared with moles, never one of each. Carbon-12 is the reference because its mass is defined as exactly 12 on this scale. Relative molecular mass is the same idea applied to a molecule: the mean mass of one molecule of the substance divided by one-twelfth the mass of one atom of carbon-12, obtained in practice by adding the relative atomic masses of every atom in the formula. A strong answer earns marks for these points. the numerator given as the average or mean mass of an atom of the element. the denominator given as one-twelfth the mass of one atom of carbon-12. relative molecular mass defined as the mean mass of a molecule on the same scale. keeping atoms with atoms or moles with moles throughout the definition. Exam tip: Write the definition as a fraction with a horizontal line so the examiner can mark each line separately.

    See it

    A flowchart starting from a node labelled Relative atomic mass, which branches to two nodes: the numerator, the weighted mean mass of an atom of the element, and the denominator, one-twelfth the mass of an atom of carbon-12. Both of these lead to a node stating that it is a ratio, so it has no units. The starting node also leads to a node describing relative molecular mass as the same idea applied to a molecule, which branches to two further nodes: the mean mass of one molecule divided by one-twelfth the mass of one atom of carbon-12, and the practical method of adding the relative atomic masses of every atom in the formula.
    It shows the two-part fraction defining relative atomic mass, its extension to relative molecular mass, and why the quantity has no units.

    Key words

    Say what each word means before you turn the card.

    relative atomic mass

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    carbon-12 scale

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    relative molecular mass

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    dimensionless ratio

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    See every definition
    relative atomic mass
    The weighted mean mass of an atom of the element divided by one-twelfth the mass of an atom of carbon-12.
    carbon-12 scale
    The scale on which the mass of an atom of carbon-12 is defined as exactly 12.
    relative molecular mass
    The mean mass of one molecule of the substance divided by one-twelfth the mass of one atom of carbon-12.
    dimensionless ratio
    A quantity that has no units because it is a ratio.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A flowchart starting from a node labelled Relative atomic mass, which branches to two nodes: the numerator, the weighted mean mass of an atom of the element, and the denominator, one-twelfth the mass of an atom of carbon-12. Both of these lead to a node stating that it is a ratio, so it has no units. The starting node also leads to a node describing relative molecular mass as the same idea applied to a molecule, which branches to two further nodes: the mean mass of one molecule divided by one-twelfth the mass of one atom of carbon-12, and the practical method of adding the relative atomic masses of every atom in the formula.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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