Chemistry 9701/23 — October/November 2018

Background Concept

Iron has atomic number 26, so neutral Fe is $1\text{s}^2\,2\text{s}^2\,2\text{p}^6\,3\text{s}^2\,3\text{p}^6\,3\text{d}^6\,4\text{s}^2$. Although the 4s subshell fills before 3d, when a transition metal atom loses electrons to form a cation, the 4s electrons are removed first because once occupied, the 3d subshell lies lower in energy than 4s.

Understanding the Question

The command word is 'give', so a single correct configuration earns the mark. The ion is $\text{Fe}^{2+}$, formed from Fe by losing two electrons.

Approach

Write the neutral atom configuration, then remove the two 4s electrons.

Step-by-Step Reasoning

Neutral Fe: $1\text{s}^2\,2\text{s}^2\,2\text{p}^6\,3\text{s}^2\,3\text{p}^6\,3\text{d}^6\,4\text{s}^2$. Removing two 4s electrons gives $1\text{s}^2\,2\text{s}^2\,2\text{p}^6\,3\text{s}^2\,3\text{p}^6\,3\text{d}^6$ — 24 electrons, correct for $\text{Fe}^{2+}$. The mark scheme shows $(4\text{s}^0)$ merely to indicate the empty 4s subshell; writing it is optional.

Key Takeaways

• 4s fills before 3d but empties before 3d when forming cations.
• Always count electrons to check the charge.

Common Mistakes

• Removing the two 3d electrons instead of the 4s electrons, giving $3\text{d}^4\,4\text{s}^2$ — this scores zero.
• Writing $3\text{d}^6$ before $4\text{s}^2$ in the neutral atom (filling order) — not relevant here since 4s is empty, but a common confusion.

Things to Be Careful About

• The superscripts must sum to 24 for $\text{Fe}^{2+}$; check your arithmetic.
• Write the full configuration — 'give the full electronic configuration' means no noble-gas shorthand like [Ar]3d⁶.

Background Concept

The sum of the oxidation numbers of all atoms in an ion equals the charge on that ion. In a polyatomic ion made of identical atoms, the charge is shared equally between them.

Understanding the Question

The $\text{S}_2^{2-}$ ion contains two sulfur atoms and carries a total charge of $-2$. The question tells you to assume both sulfur atoms have the same oxidation number.

Approach

Divide the total ionic charge by the number of atoms of the element.

Step-by-Step Reasoning

Total oxidation number for the two S atoms = $-2$. Since each S contributes equally, each sulfur has oxidation number $-2 \div 2 = -1$. This is analogous to peroxide, $\text{O}_2^{2-}$, where each oxygen is $-1$ rather than the usual $-2$.

Key Takeaways

• Oxidation numbers in an ion must sum to the ion's charge.
• Diatomic ions like $\text{S}_2^{2-}$ and $\text{O}_2^{2-}$ have per-atom oxidation numbers of half the ionic charge.

Common Mistakes

• Writing $-2$, assigning the whole ionic charge to each sulfur atom instead of dividing between the two atoms.
• Assuming sulfur is always $-2$ (its typical state in simple sulfides) without reading the ion formula.

Things to Be Careful About

• The answer is the oxidation number of each sulfur atom, as the question explicitly assumes equal oxidation numbers.

Background Concept

Sulfur dioxide released into the atmosphere dissolves in water droplets and is slowly oxidised (by $\text{O}_2$ and catalysed by $\text{NO}_x$) to sulfur trioxide/sulfuric acid. This produces acid rain (rainwater of pH below about 5).

Understanding the Question

The command words are 'state and explain': M1 is naming a consequence (acid rain); M2 is a specific harmful effect of that consequence. One consequence with one explanation is required.

Approach

Name acid rain, then give one concrete damaging effect from the mark scheme list.

Step-by-Step Reasoning

SO2 from roasting sulfide ores enters the air. It dissolves and oxidises to H2SO4, making rain acidic. Credible effects include: erosion of buildings and statues (especially limestone/calcium carbonate), acidification of lakes killing fish, leaching of nutrients or aluminium ions from soil, damage to trees/crops, and breathing difficulties in humans. Any one of these earns M2.

Key Takeaways

• SO2 → acid rain is a standard AS-level environmental link; always pair the named consequence with a specific effect.

Common Mistakes

• Stating only 'acid rain' with no explanation — M2 is lost.
• Vague answers like 'bad for the environment' or 'pollution' — not specific enough.
• Saying 'global warming' — SO2 is not a greenhouse gas responsible for warming; this scores zero.

Things to Be Careful About

• The explanation must name something the acid rain actually harms (buildings, fish, plants, soil pH), not just restate that rain becomes acidic.

Background Concept

A balanced equation has equal numbers of each atom on both sides. Balancing is done by counting atoms element by element.

Understanding the Question

The Fe and S coefficients are given (4 FeS2 → 2 Fe2O3); you must find the O2 coefficient and the SO2 coefficient.

Approach

Count S atoms first to get the SO2 coefficient, then count O atoms to get O2.

Step-by-Step Reasoning

• Sulfur: 4 FeS2 contains 8 S atoms, so 8 SO2 is needed (8 S on each side). This is M1's species plus M2's coefficient.
• Oxygen: right side has 2 × 3 = 6 O in Fe2O3 plus 8 × 2 = 16 O in SO2, total 22 O atoms. So O2 needed = 22 ÷ 2 = 11.
• Check Fe: 4 on each side. Balanced.

Key Takeaways

• Balance the element appearing in the fewest species first (S here), then handle the element appearing in several compounds (O) last.

Common Mistakes

• Writing 8O2 instead of 11O2 by forgetting the oxygen in Fe2O3.
• Writing SO3 instead of SO2 — the question states sulfur dioxide is produced.
• Changing the given coefficients (4 and 2) instead of filling the blanks.

Things to Be Careful About

• Both blanks must be correct for full marks: M1 for identifying O2 and SO2, M2 for the correct numbers 11 and 8.

Background Concept

Stoichiometry links the mass of a product to the mass of the reactant that formed it: mass → moles → mole ratio from the balanced equation → moles of target → mass.

Understanding the Question

All the FeS2 was converted to Fe2O3. We know the mass of Fe2O3 formed (33.18 g) and must work backwards to the mass of FeS2 that produced it, using the ratio 4 FeS2 : 2 Fe2O3 from part (ii).

Approach

Convert 33.18 g of Fe2O3 to moles (M1), apply the 4:2 ratio to get moles of FeS2, then multiply by Mr = 120.0 (M2).

Step-by-Step Reasoning

1. $n(\text{Fe}_2\text{O}_3) = 33.18 \div 159.6 = 0.2079$ mol.
2. Equation: $4\text{FeS}_2 + 11\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3 + 8\text{SO}_2$. The ratio FeS2 : Fe2O3 is 4 : 2 = 2 : 1, so $n(\text{FeS}_2) = 0.2079 \times 2 = 0.4158$ mol.
3. $m(\text{FeS}_2) = 0.4158 \times 120.0 = 49.89$ g.

Key Takeaways

• Always go through moles; never convert masses directly without the mole ratio.
• The ratio comes from the balanced equation you completed in (ii) — an error there is carried forward (ecf).

Common Mistakes

• Using the ratio 1:1 instead of 4:2, giving 24.95 g.
• Dividing by 2 instead of multiplying (0.2079 ÷ 2 × 120.0 = 12.47 g).
• Using Mr of FeS2 as 56 + 32 = 88 (forgetting the second sulfur atom).

Things to Be Careful About

• M2 depends on correct use of both the stoichiometric ratio and Mr = 120.0 with your M1 value; show the ×4/2 step explicitly so the method mark is visible.

Background Concept

Percentage by mass of a component = (mass of component ÷ total mass of mixture) × 100. Since gold is stated to be the only impurity, the total sample mass is FeS2 + Au.

Understanding the Question

Use the 49.89 g from (iii) (or the given 55.00 g fallback) with the 0.37 g of gold, and give the answer to exactly two significant figures.

Approach

Add the two masses to get the sample total, divide the gold mass by the total, and round to 2 s.f.

Step-by-Step Reasoning

$0.37 \div (0.37 + 49.89) = 0.37 \div 50.26 = 0.00736$, i.e. $0.736\%$, which rounds to $0.74\%$ (2 s.f.). If the fallback 55.00 g were used: $0.37 \div 55.37 \times 100 = 0.668\% \approx 0.67\%$.

Key Takeaways

• Percentage by mass needs the total mixture mass in the denominator, not just the FeS2 mass.

Common Mistakes

• Computing $0.37 \div 49.89 \times 100 = 0.74\%$ — coincidentally very close here, but the method is wrong; the total mass must include the gold.
• Giving the answer as 0.736 without rounding to 2 s.f., or rounding to 0.7 (1 s.f.).

Things to Be Careful About

• The question explicitly demands two significant figures: 0.74 %, not 0.7 % or 0.736 %.