IB DP Chemistry Mock Exam HL Paper 1B Set 2 - 2025 Syllabus

(a)
One possible set of steps (any four sensible steps gain credit):
• Use a magnet to pull out the iron filings \(\text{Fe}(s)\) from the mixture.
• Add excess water so that the sodium chloride \(\text{NaCl}(s)\) dissolves to form \(\text{NaCl}(aq)\).
• Filter the mixture so that the sand \(\text{SiO}_2(s)\) remains as the residue, while the salt solution passes through as the filtrate.
• Wash the sand with a little water and then allow or gently heat the sand to dry completely.
• Heat the salt solution to evaporate the water, or leave it to stand so that the water evaporates and dry salt crystals remain.

(b)
Total initial mass of mixture: \[ m_{\text{total}} = 5.62\ \text{g} \] Mass of iron: \[ m_{\text{Fe}} = 2.17\ \text{g} \] Mass of sand: \[ m_{\text{sand}} = 1.98\ \text{g} \] Percent composition of iron: \[ \%\,\text{Fe} = \frac{m_{\text{Fe}}}{m_{\text{total}}} \times 100 = \frac{2.17}{5.62} \times 100 \approx 38.6\ \% \] Percent composition of sand: \[ \%\,\text{sand} = \frac{m_{\text{sand}}}{m_{\text{total}}} \times 100 = \frac{1.98}{5.62} \times 100 \approx 35.2\ \% \] So the mixture is approximately \(\,38.6\ \%\) iron and \(\,35.2\ \%\) sand by mass.

(c)
One acceptable set of answers is:
Error: The sand or salt sample was still wet when weighed, so extra water increased the measured mass.
Reduce or eliminate: Heat the sample gently and re-weigh until a constant mass is obtained (dry to constant mass).

Alternative acceptable explanation:
Error: The iron filings partially oxidized to \(\text{FeO}\) or \(\text{Fe}_2\text{O}_3\) (rust), which has a greater mass than the original \(\text{Fe}\).
Reduce or eliminate: Minimize the time the iron is exposed to air and moisture, or keep it dry during the experiment.

(d)
Iron has a much higher density than sand and salt, so even a small volume of iron can have a relatively large mass. Therefore, the iron does not necessarily have the lowest percent by mass, even if it appears to be present in the smallest visible quantity.

(e)
Separation method: Fractional distillation.
Physical property: Difference in boiling points of methanol and butan-\(1\)-ol. Methanol, with the lower boiling point, vaporizes and condenses first, allowing the two liquids to be separated.