AQA · GCSE · Chemistry

    Course version: 8462 · Version from Sept 2018

    If the volumes of two solutions that react completely are known and the…

    Full specification wording

    Atom economy: If the volumes of two solutions that react completely are known and the concentration of one solution is known, the concentration of the other solution can be calculated.

    Note: This content officially belongs to section 4.3.4 Using concentrations of solutions in mol/dm³ and is strictly Higher Tier only.

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

    • Point 6 of 7
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Note: This content officially belongs to section 4.3.4 Using concentrations of solutions in mol/dm³ and is strictly Higher Tier only. Titration calculations use concentrations in mol/dm³ and balanced equations to link reacting solutions. First calculate moles of the known solution using amount = concentration × volume in dm³. Then use the mole ratio to find moles of the unknown solution. Finally divide its moles by its volume in dm³ to find its concentration. For example, 25.0 cm³ of 0.100 mol/dm³ HCl neutralises 20.0 cm³ of NaOH. Moles of HCl = 0.100 × 0.0250 = 0.00250 mol. The equation HCl + NaOH → NaCl + H₂O shows a 1:1 ratio, so moles of NaOH = 0.00250 mol. Concentration of NaOH = 0.00250 ÷ 0.0200 = 0.125 mol/dm³.

    What earns the marks

    • Calculate moles of the solution with known concentration using amount = concentration × volume in dm³.
    • Use the balanced equation to identify the mole ratio between the two reactants.
    • Apply the mole ratio to find moles of the second solution.
    • Divide moles of the second solution by its volume in dm³ to calculate its concentration.
    • Convert both volumes from cm³ to dm³ before any calculation.

    Common mistakes and how to put them right

    • Ignoring the mole ratio and assuming a 1:1 reaction.

      Put it right by writing and balancing the equation first.

    • Dividing by the volume in cm³ instead of dm³.

      Put it right by converting both volumes to dm³.

    • Using the volume of the known solution for the unknown solution.

      Put it right by keeping each solution's volume with its own calculation.

    Exam tips

    • Write the balanced equation and state the mole ratio before doing any arithmetic.
    • Label each calculated amount clearly, for example 'moles of HCl' and 'moles of NaOH'.
    • Convert all volumes to dm³ at the start and keep units with every number.

    Try the lesson

    The start of the Atom economy: If the volumes of two solutions that react completely are known and the concentration of one solution is known, the concentration of the other solution can be calculated. lesson, made from checked teaching notes for this specification point.

    Have a go first

    20.0 cm³ of 0.15 mol/dm³ sulfuric acid reacts exactly with 30.0 cm³ of potassium hydroxide solution. What is the concentration of the potassium hydroxide? (H₂SO₄ reacts with KOH in a 1:2 ratio)

    Listen

    Note: This content officially belongs to section 4.3.4 Using concentrations of solutions in mol/dm³ and is strictly Higher Tier only. Titration calculations use concentrations in mol/dm³ and balanced equations to link reacting solutions. First calculate moles of the known solution using amount = concentration × volume in dm³. Then use the mole ratio to find moles of the unknown solution. Finally divide its moles by its volume in dm³ to find its concentration. For example, 25.0 cm³ of 0.100 mol/dm³ HCl neutralises 20.0 cm³ of NaOH. Moles of HCl = 0.100 × 0.0250 = 0.00250 mol. The equation HCl + NaOH → NaCl + H₂O shows a 1:1 ratio, so moles of NaOH = 0.00250 mol. Concentration of NaOH = 0.00250 ÷ 0.0200 = 0.125 mol/dm³. A strong answer earns marks for these points. Calculate moles of the solution with known concentration using amount = concentration × volume in dm³. Use the balanced equation to identify the mole ratio between the two reactants. Apply the mole ratio to find moles of the second solution. Divide moles of the second solution by its volume in dm³ to calculate its concentration. A common slip: Ignoring the mole ratio and assuming a 1:1 reaction. Instead, writing and balancing the equation first. Exam tip: Write the balanced equation and state the mole ratio before doing any arithmetic.

    See it

    A top-to-bottom flowchart with seven boxes. The first box says the known solution's concentration and volume are given. An arrow leads to a box about converting volumes from cm³ to dm³. An arrow leads to a box about calculating moles of the known solution using amount = concentration × volume in dm³. An arrow leads to a box about writing the balanced equation and stating the mole ratio. An arrow leads to a box about applying the mole ratio to find moles of the second solution. An arrow leads to a box about dividing the moles of the second solution by its volume in dm³. A final arrow leads to a box giving the concentration of the second solution.
    The sequence of steps for calculating the concentration of an unknown solution from titration data.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Concentration of second solution
    2. 2. Divide moles of second solution by its volume in dm³
    3. 3. Calculate moles of known solution: amount = concentration × volume in dm³
    4. 4. Apply mole ratio to find moles of second solution
    5. 5. Known solution: concentration and volume
    6. 6. Convert volumes from cm³ to dm³
    7. 7. Write balanced equation and state mole ratio

    Key words

    Say what each word means before you turn the card.

    Titration calculations

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

    Mole ratio

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

    Concentration and volume

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

    Balanced equations

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

    See every definition
    Titration calculations
    Use concentrations in mol/dm³ and balanced equations to link reacting solutions, calculating the concentration of the other solution when the volumes of two solutions that react completely and the concentration of one solution are known.
    Mole ratio
    Found from the balanced equation and used to find moles of the unknown solution from the moles of the known solution.
    Concentration and volume
    Moles of the known solution are calculated using amount = concentration × volume in dm³, and the unknown concentration is found by dividing its moles by its volume in dm³.
    Balanced equations
    Used to identify the mole ratio between the two reactants, which is applied to find moles of the second solution.

    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-to-bottom flowchart with seven boxes. The first box says the known solution's concentration and volume are given. An arrow leads to a box about converting volumes from cm³ to dm³. An arrow leads to a box about calculating moles of the known solution using amount = concentration × volume in dm³. An arrow leads to a box about writing the balanced equation and stating the mole ratio. An arrow leads to a box about applying the mole ratio to find moles of the second solution. An arrow leads to a box about dividing the moles of the second solution by its volume in dm³. A final arrow leads to a box giving the concentration of the second solution.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

    There is more to this lesson

    The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.

    Carry on with the full lesson

    A week of Premium from your first lesson · Then the free plan continues · No card, no charge

    Test your understanding of Atom economy: If the volumes of two solutions that react completely are known and the concentration of one solution is known, the concentration of the other solution can be calculated.

    Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.

    Take topic quiz

    A week of Premium from your first lesson · Then the free plan continues · No card, no charge

    Start free

    Cookies on MasteryMind

    Essential cookies keep you signed in.

    Analytics and advertising cookies stay off unless you accept.

    Read our Privacy Policy.