Amount of substance exam tips

Exam tips

• Show all steps and units

  Write every step of your calculation, include the governing relationship, substitute numerical values, carry units through, and round to the correct number of significant figures.

  This demonstrates clear reasoning, prevents unit errors, and ensures the answer is presented with the required precision.

• Verify the balanced equation before calculations

  Check that all atoms and charges are balanced and that the stoichiometric coefficients are correct before proceeding with any mole calculations.

  An unbalanced equation gives wrong stoichiometric ratios, which will propagate errors through the entire calculation and lead to an incorrect limiting reagent or product mass.

• Show all steps and units

  When calculating percentage yield, write the full expression, substitute the numerical values, cancel units, and keep track of significant figures.

  Examiners look for clear reasoning and correct use of significant figures; omitting steps can lead to marks lost even if the final number is correct.

• Double-check your calculations

  Always verify your calculations for atom economy to avoid simple arithmetic errors.

  Small mistakes can lead to incorrect conclusions about the efficiency of a reaction.

• Always Show Your Working

  When calculating percentage yield or atom economy, always show your working to gain partial credit.

  Even if the final answer is incorrect, demonstrating your understanding of the calculation process can earn you marks.

• Check units and significant figures

  Write down the units of every quantity and carry them through the calculation; round the final answer to the correct number of significant figures.

  This prevents errors that can lead to incorrect answers and demonstrates careful scientific practice.

• Show All Working

  Always show your calculations step-by-step, including units and conversions.

  This helps to avoid mistakes and allows partial credit if the final answer is incorrect.

• Show all steps in titration calculations

  Write the governing relationship (n = C × V), substitute the known values, carry units through the calculation, and round the final answer to the correct number of significant figures.

  Demonstrating the full calculation process is essential for marks and ensures that the student’s reasoning is clear and correct.

• Check significant figures

  Always carry at least one extra significant figure through all intermediate calculations and round the final answer to the least precise measurement used in the problem.

  This practice ensures compliance with AQA marking criteria and avoids over‑precision that could be penalised.

• Check your calculations before the experiment

  Calculate the expected volume of titrant required using the known concentrations and volumes. If the measured volume differs significantly, re‑check the preparation of the standard solution and the accuracy of the pipette.

  Early detection of errors prevents wasted time and ensures reliable results.