AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Molecular formula is the actual number of atoms of each element in a…

    Full specification wording

    Empirical and molecular formula: Molecular formula is the actual number of atoms of each element in a compound.

    The molecular formula states how many atoms of each element are actually present in one molecule.

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

    • Point 2 of 4
    • 2 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    The molecular formula states how many atoms of each element are actually present in one molecule. Ethanoic acid is C₂H₄O₂, benzene is C₆H₆, glucose is C₆H₁₂O₆. It carries more information than the empirical formula but less than a structural formula, since it says nothing about how the atoms are joined: C₂H₆O could be ethanol or methoxymethane. Sometimes the molecular and empirical formulae are identical, as in water, methane and ammonia, because the subscripts already have no common factor. Molecular formulae are only meaningful for substances made of discrete molecules; giant ionic and giant covalent substances are described by a formula unit instead.

    What earns the marks

    • The actual number of atoms of each element in one molecule.
    • Recognising when the molecular formula and empirical formula are the same.
    • Showing the molecular formula as a whole-number multiple of the empirical formula.
    • Distinguishing a molecular formula from a structural formula.

    Common mistakes and how to put them right

    • Giving the empirical formula when the molecular formula was requested.

    • Assuming the two formulae must always differ, so doubling an already simplest formula.

    • Miscounting hydrogen atoms when reading a skeletal or displayed structure.

    • Quoting a molecular formula for an ionic compound such as sodium chloride.

    Exam tips

    • Count atoms from a displayed formula systematically, element by element, rather than by eye.
    • State whether the question wants empirical, molecular or structural before you write anything.
    • Remember that isomers share a molecular formula, so a molecular formula alone never identifies a compound.

    Try the lesson

    The start of the Empirical and molecular formula: Molecular formula is the actual number of atoms of each element in a compound. lesson, made from checked teaching notes for this specification point.

    Have a go first

    A compound has the structural formula HOOCCH₂CH₂COOH. What is the molecular formula of this compound?

    Listen

    The molecular formula states how many atoms of each element are actually present in one molecule. Ethanoic acid is C₂H₄O₂, benzene is C₆H₆, glucose is C₆H₁₂O₆. It carries more information than the empirical formula but less than a structural formula, since it says nothing about how the atoms are joined: C₂H₆O could be ethanol or methoxymethane. Sometimes the molecular and empirical formulae are identical, as in water, methane and ammonia, because the subscripts already have no common factor. Molecular formulae are only meaningful for substances made of discrete molecules; giant ionic and giant covalent substances are described by a formula unit instead. A strong answer earns marks for these points. the actual number of atoms of each element in one molecule. recognising when the molecular formula and empirical formula are the same. showing the molecular formula as a whole-number multiple of the empirical formula. distinguishing a molecular formula from a structural formula. Exam tip: Count atoms from a displayed formula systematically, element by element, rather than by eye.

    See it

    A top-down flowchart starting from a single node labelled 'Molecular formula: actual number of atoms of each element in a compound'. Four branches lead from it. The first branch goes to 'Carries more information than empirical formula'. The second goes to 'Carries less information than structural formula', which leads on to 'Says nothing about how the atoms are joined', which leads to 'C2H6O could be ethanol or methoxymethane'. The third branch goes to 'Sometimes molecular and empirical formulae are identical', which leads to 'Water, methane and ammonia: subscripts have no common factor'. The fourth branch goes to 'Only meaningful for substances made of discrete molecules', which leads to 'Giant ionic and giant covalent substances described by a formula unit instead'.
    It shows what the molecular formula tells you, how it compares with the empirical and structural formulae, when it is identical to the empirical formula, and when it applies.

    Key words

    Say what each word means before you turn the card.

    molecular formula

    Try answering aloud or in your head before turning the card.

    isomers

    Try answering aloud or in your head before turning the card.

    See every definition
    molecular formula
    The actual number of atoms of each element in a compound, stating how many atoms of each element are actually present in one molecule.
    isomers
    Compounds that share a molecular formula, so a molecular formula alone never identifies a compound.

    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 top-down flowchart starting from a single node labelled 'Molecular formula: actual number of atoms of each element in a compound'. Four branches lead from it. The first branch goes to 'Carries more information than empirical formula'. The second goes to 'Carries less information than structural formula', which leads on to 'Says nothing about how the atoms are joined', which leads to 'C2H6O could be ethanol or methoxymethane'. The third branch goes to 'Sometimes molecular and empirical formulae are identical', which leads to 'Water, methane and ammonia: subscripts have no common factor'. The fourth branch goes to 'Only meaningful for substances made of discrete molecules', which leads to 'Giant ionic and giant covalent substances described by a formula unit instead'.

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