Using Moles

    OCR
    GCSE

    The mole is the fundamental SI unit for the amount of substance, defined by the Avogadro constant (6.02 x 10^23 particles). It serves as the critical bridge between the atomic scale and macroscopic measurements, enabling the calculation of reacting masses via the relationship Mass = Moles x Molar Mass. Mastery involves applying stoichiometry to balanced equations to determine theoretical yields, identifying limiting reactants, and calculating concentrations in solutions. This quantitative framework underpins all analytical chemistry and industrial process design.

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    Objectives
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    Exam Tips
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    Pitfalls
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    Key Terms
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    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for correctly calculating the relative formula mass (Mr) using relative atomic masses from the Periodic Table
    • Award 1 mark for correct substitution into the equation: moles = mass / Mr
    • Credit responses that apply the correct stoichiometric ratio from the balanced symbol equation to determine reacting moles
    • Award 1 mark for calculating the final mass or concentration, ensuring the answer is given to the specified number of significant figures
    • For limiting reactant questions, award marks for explicitly comparing the calculated moles of reactants against the stoichiometric ratio

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You have correctly calculated the moles, but check your significant figures in the final answer."
    • "Remember to convert volume to dm³ when calculating concentration; dividing by 1000 is a crucial step."
    • "Your method for finding the limiting reactant is valid, but you must explicitly state which chemical limits the reaction."
    • "Excellent use of the mole ratio. To ensure full marks, show the rearrangement of the formula before substituting values."

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for correctly calculating the relative formula mass (Mr) using relative atomic masses from the Periodic Table
    • Award 1 mark for correct substitution into the equation: moles = mass / Mr
    • Credit responses that apply the correct stoichiometric ratio from the balanced symbol equation to determine reacting moles
    • Award 1 mark for calculating the final mass or concentration, ensuring the answer is given to the specified number of significant figures
    • For limiting reactant questions, award marks for explicitly comparing the calculated moles of reactants against the stoichiometric ratio

    Examiner Tips

    Expert advice for maximising your marks

    • 💡Always show full working out; OCR mark schemes frequently award 'error carried forward' (ECF) marks if the method is correct but a calculation error occurred earlier
    • 💡When determining the limiting reactant, explicitly state which reactant is in excess and which limits the reaction based on the mole ratio, not just the starting mass
    • 💡Pay close attention to the command to give answers to a specific number of significant figures; failing to do so often results in the loss of the final mark

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Using atomic numbers instead of relative atomic masses when calculating Mr
    • Failing to convert volumes from cm³ to dm³ before calculating concentration in mol/dm³
    • Rounding intermediate values in multi-step calculations, leading to cumulative errors in the final answer
    • Confusing the large stoichiometric coefficient with the subscript numbers when calculating molar mass

    Key Terminology

    Essential terms to know

    The Mole and Avogadro Constant
    Relative Formula Mass (Mr) and Molar Mass
    Reacting Masses and Stoichiometry
    Limiting Reactants
    Concentration of Solutions (mol/dm3)

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Explain
    Deduce
    Determine
    Show

    Practical Links

    Related required practicals

    • •{"code":"PAG 1","title":"Moles and equations","relevance":"Determination of the composition of magnesium oxide or similar empirical formula experiments"}
    • •{"code":"PAG 2","title":"Acid-base titration","relevance":"Using titration results to calculate the concentration of a solution using mole ratios"}

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