A-Level · Chemistry · AQA · Mark scheme decoded
AQA A-Level Chemistry: The Avogadro Constant and Mole Calculations — mark scheme explained
The short answer
The concept of the mole is fundamental in chemistry, providing a bridge between the microscopic world of atoms and molecules and the macroscopic quantities we can measure in the laboratory.
The question
Calculate the number of molecules in 0.5 moles of water (H 2 O).
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For calculation questions, marks are typically awarded for correct substitution of values into formulas, accurate arithmetic, and providing the final answer with appropriate units. For conceptual questions, marks are given for clear and concise explanations that demonstrate a deep understanding of the topic.
What the command words demand
- Calculate
- Perform a numerical calculation using given data and appropriate formulas.
- Determine
- Find or derive a specific value or quantity based on given information.
- Explain
- Provide a detailed description of the concept, including relevant principles and relationships.
- Convert
- Change one unit of measurement to another using conversion factors.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, perform calculations, and check your work.
- Identify the given values.0 marksn = 0.5 molesN A = 6.022 × 10 23 mol -1
- Use the formula for the number of particles.1 markN = n × N A
- Substitute the values into the formula.1 markN = 0.5 × 6.022 × 10 23
- Perform the multiplication.1 markN ≈ 3.011 × 10 23
Final answer: 3.011 × 10 23 molecules
Work through every step correctly and you earn all 3 marks.
Another worked example
Calculate the amount of sodium chloride (NaCl) in moles if you have 58.44 grams of NaCl.
- Identify the given values.0 marksm = 58.44 gM NaCl = 58.44 g mol -1
- Use the formula for converting mass to moles.1 markn = m / M
- Substitute the values into the formula.1 markn = 58.44 g / 58.44 g mol -1
- Perform the division.1 markn = 1 mole
Final answer: 1 mole
Work through every step correctly and you earn all 3 marks.
Common mistakes
Using the wrong units for mass and volume.
Why it happens: Students often forget to convert units to standard form (grams for mass and cubic decimeters for volume) before using them in calculations.
Fix: Always ensure that mass is in grams and volume is in cubic decimeters when substituting into the formulas.
Forgetting to use the Avogadro constant in particle calculations.
Why it happens: The Avogadro constant can be easily overlooked, leading to incorrect calculations of the number of particles.
Fix: Always include the Avogadro constant (N A ) when calculating the number of particles using the formula N = n × N A .
Confusing molar mass with molecular weight.
Why it happens: Students sometimes mix up the terms molar mass and molecular weight, leading to incorrect conversions from mass to moles.
Fix: Remember that molar mass is the mass of one mole of a substance in grams per mole (g mol -1 ). Use this value when converting mass to moles using the formula n = m / M.
Incorrectly calculating concentration from volume and amount.
Why it happens: Students may use the wrong formula or make arithmetic errors when calculating concentration (C = n / V).
Fix: Always use the correct formula for concentration: C = n / V. Double-check your calculations to ensure accuracy.
Not reporting results with the appropriate number of significant figures.
Why it happens: Students often forget to consider the precision of their measurements when reporting final answers.
Fix: Always report your final answer with the same number of significant figures as the least precise measurement in your calculations.
Failing to convert volume from cm 3 to dm 3 .
Why it happens: Students may forget that 1 dm 3 is equal to 1000 cm 3 , leading to incorrect concentration calculations.
Fix: Always convert volume from cubic centimeters (cm 3 ) to cubic decimeters (dm 3 ) by dividing by 1000 when using the formula C = n / V.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Calculate Number Of Molecules | Use N = n × Nₐ to find the number of molecules in a given amount. | 3 |
| Moles From Mass | Convert a given mass of NaCl into an amount in moles using its molar mass. | 3 |
| Calculate Concentration | Find the molar concentration of a solution from moles of solute and its volume. | 3 |
| Calculate Moles | Convert volume to dm³ and use concentration to find the amount of solute in moles. | 3 |
| Total across these question types | 12 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.