VCE Chemistry significant figures & data book — stop losing marks

In VCE Chemistry, calculation questions are where careful students either secure marks or quietly leak them. The chemistry can be completely correct and still lose the final mark to a significant-figure slip, a missing unit, or dividing by the wrong quantity. Add in using the provided data book well, and you have a set of habits that are worth several marks across a paper.

Significant figures and units

Match your final answer's significant figures to the data given — if the question quotes concentrations to three figures, your answer should be to three. Carry units through every line of a calculation, and keep full precision in intermediate steps so rounding only happens at the end. These are the same disciplines as Physics, and in Chemistry the answer mark often depends on them explicitly.

Use the data book like it's part of the exam

The VCAA data book is provided, so the skill is using it fluently, not memorising constants. Know where the molar masses, standard potentials and constants live so you aren't hunting under time, and pull values from it rather than half-remembering them. When a question references a table, state which value you used and why.

A worked example: the wrong volume

Question (3 marks): In a titration, 20.0 mL of HCl is exactly neutralised by 25.0 mL of 0.100 M NaOH. Calculate the concentration of the HCl.

A student writes: n(NaOH) = 0.100 × 0.0250 = 2.50 × 10⁻³ mol; n(HCl) = 2.50 × 10⁻³ mol; c = 2.50 × 10⁻³ / 0.0250 = 0.100 M.

Marking (illustrative, not an official VCAA scheme): finding n(NaOH) earns a method mark, and applying the 1:1 mole ratio earns the second. The final mark is lost — the student divided by the NaOH volume (0.0250 L) instead of the HCl volume. Using V(HCl) = 0.0200 L gives c = 2.50 × 10⁻³ / 0.0200 = 0.125 M.

The habit that prevents this: label each volume with the substance it belongs to, and at the final step divide by the volume of the substance you are actually finding.