AQA · GCSE · Chemistry

    Course version: 8462 · Version from Sept 2018

    Students should understand the use of the multipliers in equations in…

    Full specification wording

    Conservation of mass and balanced chemical equations: Students should understand the use of the multipliers in equations in normal script before a formula and in subscript within a formula.

    Chemical formulae use two different multiplier positions with distinct meanings.

    Checked against the AQA 8462 specification (opens in a new tab) on .

    • Point 3 of 3
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Chemical formulae use two different multiplier positions with distinct meanings. A subscript, written small and low after a symbol, multiplies only the atom immediately before it within that formula unit: H₂O contains two hydrogen atoms and one oxygen atom, and Ca(OH)₂ contains one calcium, two oxygen and two hydrogen atoms because the bracket groups the OH unit. A coefficient in normal script before a formula multiplies the whole formula unit: 2H₂O means two water molecules, giving four hydrogen atoms and two oxygen atoms in total. When counting atoms in an equation, apply subscripts first, then coefficients. For example, 2Ca(NO₃)₂ contains two calcium atoms, four nitrogen atoms and twelve oxygen atoms. This distinction underpins balancing equations and all mole and mass calculations.

    What earns the marks

    • A subscript applies only to the atom or bracketed group immediately before it, not to earlier elements in the formula.
    • A coefficient in normal script before a formula multiplies every atom in that formula unit.
    • When counting atoms, evaluate subscripts within the formula before applying any coefficient.
    • Brackets with a subscript multiply every element inside the bracket, as in Ca(OH)₂.
    • Correct interpretation of multipliers is needed to balance equations and to calculate relative formula masses.
    • In 2Ca(NO₃)₂ the coefficient doubles the whole formula, giving 2 Ca, 4 N and 12 O atoms.

    Common mistakes and how to put them right

    • Treating a subscript as multiplying the whole formula, so reading H₂O as four hydrogen atoms.

      Put it right: The subscript multiplies only the preceding symbol, giving two hydrogen atoms.

    • Applying a coefficient to only the first element of a formula, so reading 2CO₂ as two carbon and two oxygen atoms.

      Put it right: The coefficient multiplies every atom, giving two carbon and four oxygen atoms.

    • Ignoring brackets when counting, so reading Ca(OH)₂ as one oxygen and one hydrogen.

      Put it right: The subscript after the bracket multiplies each element inside, giving two oxygen and two hydrogen atoms.

    Exam tips

    • Underline or circle each coefficient and subscript before counting atoms so you do not confuse their roles.
    • Write out an atom tally for each side of an equation; this makes balancing errors visible and easy to correct.
    • Check bracketed groups separately, multiplying each element inside by the subscript outside the bracket.
    • If a count seems odd, re-read the formula from left to right, applying subscripts before coefficients.

    Try the lesson

    The start of the Conservation of mass and balanced chemical equations: Students should understand the use of the multipliers in equations in normal script before a formula and in subscript within a formula. lesson, made from checked teaching notes for this specification point.

    Have a go first

    In the formula 3CO₂, what does the large number 3 in front of the formula tell you?

    Listen

    Chemical formulae use two different multiplier positions with distinct meanings. A subscript, written small and low after a symbol, multiplies only the atom immediately before it within that formula unit: H₂O contains two hydrogen atoms and one oxygen atom, and Ca(OH)₂ contains one calcium, two oxygen and two hydrogen atoms because the bracket groups the OH unit. A coefficient in normal script before a formula multiplies the whole formula unit: 2H₂O means two water molecules, giving four hydrogen atoms and two oxygen atoms in total. When counting atoms in an equation, apply subscripts first, then coefficients. For example, 2Ca(NO₃)₂ contains two calcium atoms, four nitrogen atoms and twelve oxygen atoms. This distinction underpins balancing equations and all mole and mass calculations. A strong answer earns marks for these points. A subscript applies only to the atom or bracketed group immediately before it, not to earlier elements in the formula. A coefficient in normal script before a formula multiplies every atom in that formula unit. When counting atoms, evaluate subscripts within the formula before applying any coefficient. Brackets with a subscript multiply every element inside the bracket, as in Ca(OH)₂. A common slip: Treating a subscript as multiplying the whole formula, so reading H₂O as four hydrogen atoms. Instead, the subscript multiplies only the preceding symbol, giving two hydrogen atoms. Exam tip: Underline or circle each coefficient and subscript before counting atoms so you do not confuse their roles.

    See it

    A top-down flowchart. The top node, 'Formula with multipliers', branches into two nodes: 'Subscript: small, low, after a symbol' and 'Coefficient: normal script before a formula'. The subscript node leads to 'Multiplies only the atom or bracketed group immediately before it'; the coefficient node leads to 'Multiplies every atom in the whole formula unit'. Both of these lead to 'Apply subscripts first', which leads to 'Then apply coefficients', which leads to 'Count atoms of each element'.
    It shows the two multiplier positions in a formula, what each one multiplies, and the order of applying them when counting atoms.

    Key words

    Say what each word means before you turn the card.

    Subscript

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    Coefficient

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    Brackets

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    Counting atoms

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    See every definition
    Subscript
    A small, low number written after a symbol that multiplies only the atom or bracketed group immediately before it within that formula unit.
    Coefficient
    A number in normal script before a formula that multiplies the whole formula unit, so every atom in that formula unit is multiplied.
    Brackets
    A grouping in a formula where a subscript after the bracket multiplies every element inside the bracket.
    Counting atoms
    Applying subscripts within the formula first and then coefficients, so each element's atoms in the formula unit are multiplied correctly.

    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. The top node, 'Formula with multipliers', branches into two nodes: 'Subscript: small, low, after a symbol' and 'Coefficient: normal script before a formula'. The subscript node leads to 'Multiplies only the atom or bracketed group immediately before it'; the coefficient node leads to 'Multiplies every atom in the whole formula unit'. Both of these lead to 'Apply subscripts first', which leads to 'Then apply coefficients', which leads to 'Count atoms of each element'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

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