A chemical label can contain a correct percentage and still encourage an incorrect conclusion. If two mineral salts contain different amounts of the same element by mass, it does not follow that the salt with the larger percentage is better for a plant, safer for a school experiment or equivalent in every chemical reaction. In G3 Chemistry, a formula tells you which particles are present and in what ratio. The exam skill is to follow that information carefully and stop where the evidence stops.
This learner’s guide begins a fresh mineral-label examination case after the school-garden EMS approval hearing in Volume 1144. The earlier hearing asked whether acidity observations, a drawing and a public poster supported approval. Here we examine a different kind of claim: what two compound formulae do and do not establish. All labels and sample masses are invented for calculation practice. These are not instructions to buy, mix, apply, consume or dispose of real mineral products.
- Distinguish an element’s mass percentage from the mass percentage of an entire compound.
- Derive neutral ionic formulae from charges rather than guessing subscripts.
- Calculate relative formula mass, moles and theoretical composition accurately.
- Interpret different ions and solution concentrations without treating salts as interchangeable.
- Separate mathematically valid composition claims from unproven performance and safety claims.
The 2027 G3 SEC Chemistry context
The 2027 Singapore-Cambridge Secondary Education Certificate G3 Chemistry syllabus is K324. Its compulsory papers are Paper 1 Multiple Choice, one hour, 40 marks, 30%; Paper 2 Structured and Free Response, one hour 45 minutes, 80 marks, 50%; and Paper 3 Practical, one hour 50 minutes, 40 marks, 20%. Paper 2 contains a compulsory structured section and a choice in the final section. Students must use the actual instructions on their paper, not a remembered arrangement from a practice worksheet.
The syllabus includes chemical formulae and equations, the mole concept, relative atomic and formula mass, percentage by mass of an element in a compound, solution concentrations, salts and qualitative analysis. It also emphasises handling new data, evaluating explanations and using evidence. This original label case draws on those outcomes. It is not a past paper or a predicted 2027 question.
Different G3 Science combinations have different codes and assessment schemes. If you are entered for a combined Science paper rather than standalone Chemistry, consult the correct official subject syllabus. The label-audit reasoning may still support learning, but its position in the examination must be checked against the candidate’s registered subject.
The new case: two samples for an exhibition
A science club is preparing a display on how to read a chemical ingredient label. The first fictional card is Sample A: pure, anhydrous magnesium sulfate, MgSO4. The second is Sample B: pure, anhydrous magnesium chloride, MgCl2. For this exercise only, take relative atomic masses Mg 24.3, S 32.1, O 16.0 and Cl 35.5.
The display has three competing claims. Claim one says Sample B contains a larger percentage of magnesium by mass than Sample A. Claim two says equal masses of the two samples must contain equal amounts of magnesium. Claim three says Sample B will therefore produce better garden results. The science committee has asked for an evidence audit before the public reads the cards.
The critical design detail is the word anhydrous. These particular worked calculations treat the chemical formulae as written, without additional water incorporated in a crystal. A real package may have a different precise formula, formulation, purity or water content. A calculation for an anhydrous ideal substance is not automatically transferable to an unspecified commercial product.
Begin with ions and electrical neutrality
Magnesium commonly forms Mg2+ ions. Chloride is Cl−. A neutral formula unit of magnesium chloride therefore contains one magnesium ion for every two chloride ions, giving MgCl2. The subscript two belongs to chloride, not to the magnesium charge. It records the number ratio needed to balance the charges.
Sulfate is the polyatomic ion SO4 2−. One Mg2+ and one SO4 2− balance, giving MgSO4. The four oxygen atoms belong within the sulfate group. It would be incorrect to write MgSO42 or to treat the formula as four separate oxide ions. The chemical groups must remain intact when deriving a formula.
When you check an ionic formula, count total positive and negative charges. MgCl2 has +2 from magnesium and 2 × (−1) from chloride, giving zero. MgSO4 has +2 and −2, again zero. Charge balance is an independent check on formula writing, useful before any calculation begins.
What the formula counts, and what it does not
The formula MgCl2 describes a magnesium-to-chloride ion ratio of 1:2. It does not tell you that a jar contains one magnesium atom and two chlorine atoms in total. A macroscopic sample contains enormously many formula units. Similarly, MgSO4 gives the simplest ionic ratio within the compound, not an isolated molecular arrangement like a small covalent molecule.
A chemical formula is not a full safety label. It does not tell you all the possible impurities, the source, packaging, hazards, intended use or concentration in an actual product. If a task gives extra information, use it. If it does not, do not invent a specification.
Strong candidates keep formula facts separate from product facts. The symbol is enough for a theoretical formula-mass calculation. It is not enough for claiming that two products can be substituted safely or that a particular treatment will achieve a biological outcome.
Relative formula mass of Sample A
Using the atomic masses provided, MgSO4 has relative formula mass 24.3 + 32.1 + 4 × 16.0 = 120.4. Begin by writing the formula and counting the oxygen atoms. A common mistake is to add only one oxygen mass because the student sees one sulfate group. The sulfate group contains four oxygen atoms.
The result 120.4 has no unit as a relative formula mass. Its corresponding molar mass can be expressed as 120.4 g/mol for these atomic-mass values. Do not attach grams directly to the relative formula mass as though the dimensionless sum were the mass of a sample.
Reverse-check the subtotal: magnesium and sulfur contribute 56.4; the four oxygen atoms contribute 64.0; together they give 120.4. This simple grouping catches accidental omission of the subscript.
Relative formula mass of Sample B
MgCl2 has relative formula mass 24.3 + 2 × 35.5 = 95.3. The subscript two changes the contribution from chlorine but not the charge notation. Using 24.3 + 35.5 would describe the wrong atom count and produce 59.8, which is not the formula mass of MgCl2.
The corresponding molar mass is 95.3 g/mol. For equal numbers of moles, magnesium sulfate and magnesium chloride therefore have different total masses because the other ions contribute different amounts. For equal masses, the amounts in moles are different.
This distinction is the heart of the forthcoming label audit. Always ask what is being held equal: mass of compound, amount in moles, volume of solution or amount of magnesium? A change in the chosen basis changes the comparison.
Calculate the percentage of magnesium in MgSO4
Magnesium contributes 24.3 of the 120.4 relative formula-mass units. The theoretical magnesium mass percentage is therefore (24.3 ÷ 120.4) × 100% ≈ 20.18%, or about 20.2% to three significant figures.
Interpret the answer correctly: in an ideal pure anhydrous sample of MgSO4, magnesium accounts for about one-fifth of the sample’s mass. The remaining mass comes from sulfur and oxygen in the sulfate ions. This is not a statement about the amount of dissolved magnesium available under real soil conditions.
A quick bound check helps. Since 24.3 is close to one-fifth of 120.4, a result near 20% is sensible. A result of 201.8% is impossible for a component of a pure compound and signals a division or percentage error.
Calculate the percentage of magnesium in MgCl2
Magnesium contributes 24.3 of 95.3 relative formula-mass units. Thus the theoretical magnesium mass percentage is (24.3 ÷ 95.3) × 100% ≈ 25.50%, or 25.5% to three significant figures.
Claim one is supported under the stated assumption of pure anhydrous compounds: Sample B has a higher magnesium mass percentage than Sample A. That comparison is about composition. It does not claim greater magnesium solubility, biological uptake, safety, suitability or value for money.
The size of the difference can be expressed as about 25.5% − 20.2% = 5.3 percentage points. Percentage points and percentage relative increase are different descriptions. In this science problem, stating the two percentages is often the least ambiguous form.
Equal sample masses: the claim that fails
Take 12.04 g of Sample A, MgSO4. Since its molar mass is 120.4 g/mol, the amount is 12.04/120.4 = 0.100 mol of formula units. There is one magnesium ion per formula unit, so this corresponds to 0.100 mol of magnesium ions if the compound is considered in terms of its ionic constituents.
Now take 12.04 g of Sample B, MgCl2. Divide by 95.3 g/mol to obtain approximately 0.1263 mol of formula units and therefore 0.1263 mol of magnesium-ion equivalents. Equal compound masses do not yield equal amounts of magnesium in this comparison.
Claim two is therefore false. The denominator matters. The student should not simply say “the compounds are different”. Show which difference changes the result: the molar masses are 120.4 and 95.3 g/mol, while each formula unit contains one magnesium ion.
Equal amounts of magnesium: reverse the problem
Suppose the display instead wants both samples to represent 0.100 mol of magnesium. Each formula contains one magnesium ion, so 0.100 mol of MgSO4 and 0.100 mol of MgCl2 are needed on paper. Their masses would be 12.04 g and 9.53 g respectively.
The compound masses are different, yet the magnesium amounts are the same. This is an excellent reverse problem because it tests whether the candidate understands the meaning of moles rather than memorising percentages.
One can also compare magnesium mass directly: 0.100 mol × 24.3 g/mol = 2.43 g magnesium in either ideal sample. A student who multiplies the compound mass by the correct composition percentage will recover approximately this value, subject to rounding. Two independent routes to the same answer are a useful check.
Why a mass calculation is not a growth experiment
The fictional advertising claim says, “This salt gives better plants because it has more magnesium by mass.” The calculation only supports a compositional statement. It does not supply any actual growth observations, dose-response data, controlled comparison, soil chemistry, species information or evidence that magnesium is the limiting factor.
The chloride and sulfate ions are different chemical species. Their identities can matter in a wider application, but the problem has not supplied enough evidence to predict how a plant would respond to either product. A scientist should not pretend that a higher fraction of one element settles all other variables.
A better exhibition sentence is: “The pure anhydrous formulae predict approximately 20.2% magnesium by mass for MgSO4 and 25.5% for MgCl2. This composition calculation does not establish which material is suitable for use in soil.” It communicates what is known and what remains outside the calculation.
Solubility, ions and electrical conduction
When an ionic substance dissolves in water, mobile ions can carry electrical charge. For a simplified ideal dissociation model, MgSO4(aq) separates into Mg2+(aq) and SO4 2−(aq), while MgCl2(aq) yields Mg2+(aq) and two Cl−(aq) per formula unit. The different ratios are central to interpreting concentrations.
A solid ionic lattice generally does not conduct electricity through mobile ions because the ions are fixed in position. In an aqueous solution, movement of ions allows electrical conduction. If a question asks for a particle-level explanation, name the mobile charge carriers rather than saying vaguely that the water “contains electricity”.
Real concentrations, equilibrium and measurement conditions can matter to quantitative behaviour. At this level the practice question explicitly uses an ideal dissociation model, so students should solve the given model accurately without adding unsupported claims about every real system.
Concentration example: equal moles, different ions
A fictional laboratory worksheet dissolves 0.100 mol of pure MgSO4 and makes the final solution volume up to 0.500 dm3. Its formula-unit concentration is 0.100/0.500 = 0.200 mol/dm3. In the simplified dissociation model, the magnesium-ion concentration is 0.200 mol/dm3 and the sulfate-ion concentration is also 0.200 mol/dm3.
In a separate flask, 0.100 mol MgCl2 is made up to the same final volume. Its formula-unit concentration is likewise 0.200 mol/dm3, and its magnesium-ion concentration is 0.200 mol/dm3. However, the chloride-ion concentration is 0.400 mol/dm3 because each formula unit contributes two chloride ions.
A candidate who reports 0.200 mol/dm3 chloride because the compound solution is 0.200 mol/dm3 has omitted the subscript. Another candidate might double the magnesium concentration because magnesium carries a 2+ charge. Electrical charge and particle count are different; the 2+ notation is not a coefficient multiplying the number of magnesium ions.
Volume conversion: litres and cubic decimetres
One cubic decimetre is one litre, and 1000 cubic centimetres form one cubic decimetre. Therefore 500 cm3 is 0.500 dm3, not 500 dm3. Substituting 500 rather than 0.500 into a concentration equation would produce an answer one thousand times smaller.
Before any solution calculation, write amount = concentration × volume with compatible units. If concentration is in mol/dm3, convert the volume into dm3. If it is in g/dm3, keep track of whether you are finding mass of the compound or mass of a particular element within it.
Do not skip the word “final” in the phrase “made up to a final volume”. Adding a sample to 500 cm3 of water is not automatically the same as preparing a final solution volume of 500 cm3. A careful reader distinguishes the quantity of solvent added from the volume of the completed solution.
The difference between g/dm3 and mol/dm3
The two concentration units describe different quantities. For an ideal pure MgSO4 solution at 0.200 mol/dm3, the corresponding compound mass concentration is 0.200 × 120.4 = 24.08 g/dm3. For MgCl2 at 0.200 mol/dm3, it is 0.200 × 95.3 = 19.06 g/dm3.
Notice the result: equal molar concentrations of these two compounds do not correspond to equal compound mass concentrations. Nor do equal mass concentrations imply equal molar concentrations. The conversion requires the appropriate molar mass.
Some questions ask for a concentration of magnesium ions rather than the compound. That is a third possible meaning. Always label the chemical entity beside the number. “0.200 mol/dm3” is incomplete if the reader does not know which substance or ion it refers to.
An experiment cannot identify a chemical from a percentage alone
Suppose an unlabeled powder produces a magnesium mass percentage of around 20% in a hypothetical analytical report. That is consistent with the theoretical mass fraction of pure anhydrous MgSO4 under our chosen atomic masses, but it does not prove the powder is MgSO4. Mixtures and other substances can share a similar overall percentage.
Identification requires relevant chemical tests and supporting observations. In the syllabus, qualitative analysis includes controlled tests for selected cations, anions and gases. A single number is not a substitute for a set of discriminating observations.
A careful answer says “consistent with” when the evidence is consistent but not unique, and reserves “identified as” for the circumstances the task actually establishes. In a data-based paper, that precision can separate a defensible inference from an attractive overstatement.
Hydrated products and why exact labels matter
The word “anhydrous” in this exercise means that the formula used in the calculation does not include water of crystallisation. Some real substances are supplied as hydrates, with water represented in their chemical formula. If the exact compound changes, the total formula mass and elemental mass percentages change.
This guide does not require students to memorise a particular commercial hydrate formula or to assume every product is anhydrous. The important transfer skill is to use the actual formula given in the question. A label reading only “magnesium sulfate” may be insufficient for applying our specific 20.2% example to a product without further details.
Do not invent missing formula information merely to make a calculation possible. In a national exam, when information is needed for an unfamiliar calculation, it should be supplied or derivable using the syllabus and periodic table. State the assumption when a teaching example makes it explicit.
Practical skill: plan a defensible comparison
If a class wishes to investigate two labelled laboratory samples, the first step is to define the measured variable. Is the aim to compare conductivity at an equal formula-unit concentration, identify an ion, or measure something else? Each requires a different design and different controls. “Which one is best?” is not a well-defined scientific question.
For a conductivity comparison, identify the concentration basis, final volume, temperature, apparatus and measurement method. Do not claim the result follows only from counting formula ions; actual mobility and conditions also affect conductivity. A fair design controls relevant variables and records observations before explaining them.
For an identity question, select tests whose observations discriminate between possible ions, following official laboratory procedures and supervision. A school chemistry activity is not a recommendation for unsupervised mixing or testing of consumer products. The exercise is about scientific planning and interpreting information, not conducting experiments at home.
Worked errors: six misconceptions
Error one: writing MgCl instead of MgCl2. Repair: balance the +2 magnesium charge with two −1 chloride ions. Error two: calculating MgSO4 as 24.3 + 32.1 + 16.0. Repair: include four oxygen atoms. Error three: giving 120.4 g for relative formula mass. Repair: the relative quantity is dimensionless; 120.4 g/mol is the molar mass.
Error four: saying 25.5% magnesium is 25.5% more magnesium than 20.2%. Repair: those figures differ by 5.3 percentage points; a relative comparison would use the first value as a denominator and answer a different question.
Error five: assuming equal grams means equal moles. Repair: divide each mass by its own molar mass. Error six: assuming a higher theoretical elemental fraction guarantees better practical results. Repair: identify the absent outcomes and conditions rather than turning composition into performance.
These errors are useful because each has a different cause. Formula writing, atom counting, unit labelling, percentage interpretation, mole conversion and evidence appraisal should each receive its own targeted practice. An undifferentiated instruction to “be careful” cannot repair all six.
Mini-mock A: formulae and relative mass
Question 1: Given Mg2+ and Cl−, write a neutral compound formula. Answer: MgCl2, because two chloride ions are needed per magnesium ion. The charge check is +2 + 2(−1) = 0.
Question 2: Calculate the relative formula mass of MgSO4. Answer: 24.3 + 32.1 + 4 × 16.0 = 120.4. It is not written as 120.4 g.
Question 3: Calculate the relative formula mass of MgCl2. Answer: 24.3 + 2 × 35.5 = 95.3.
Question 4: State the ratio of magnesium ions to chloride ions represented by MgCl2. Answer: 1:2. The compound contains a magnesium cation and two chloride anions in its simplest ionic ratio.
Mini-mock B: elemental percentages
Question 5: What percentage of the mass of pure anhydrous MgSO4 is magnesium? Answer: (24.3/120.4) × 100 ≈ 20.2% to three significant figures.
Question 6: What percentage of pure anhydrous MgCl2 is magnesium? Answer: (24.3/95.3) × 100 ≈ 25.5% to three significant figures.
Question 7: Is the difference 5.3% or 5.3 percentage points? Answer: approximately 5.3 percentage points. A relative percentage increase needs a separately stated comparison base.
Question 8: A poster says “MgCl2 contains more magnesium by mass than MgSO4”. Under the pure anhydrous formula assumptions, is that supported? Yes. But a claim that it therefore performs better in a garden is not supported without additional evidence.
Mini-mock C: moles and masses
Question 9: Calculate the amount in moles of 12.04 g MgSO4. Answer: 12.04/120.4 = 0.100 mol.
Question 10: What mass of MgCl2 contains 0.100 mol of the pure compound? Answer: 0.100 × 95.3 = 9.53 g.
Question 11: What mass of magnesium is theoretically present in 0.100 mol of either compound? Answer: 0.100 × 24.3 = 2.43 g, because each contains one magnesium per formula unit.
Question 12: Explain why a 12.04 g sample of MgCl2 does not contain the same amount of magnesium as 12.04 g MgSO4. Answer: the samples contain different amounts of formula units because the molar masses are different. The MgCl2 sample contains about 0.126 mol whereas the MgSO4 sample contains 0.100 mol.
Mini-mock D: concentration and ions
Question 13: A 0.100 mol MgSO4 sample is made into 0.500 dm3 of solution. Calculate its formula-unit concentration. Answer: 0.200 mol/dm3.
Question 14: In the simplified fully dissociated model, what is the Mg2+ concentration in that solution? Answer: 0.200 mol/dm3, because one formula unit contributes one Mg2+.
Question 15: A 0.100 mol MgCl2 sample is made into 0.500 dm3. What is the chloride-ion concentration? Answer: 0.400 mol/dm3, two times the formula-unit concentration.
Question 16: Convert 500 cm3 to dm3. Answer: 0.500 dm3. The factor of 1000 must be applied before using mol/dm3.
Mini-mock E: evidence and communication
Question 17: Why is a product’s exact chemical formula relevant? Answer: formula mass, elemental fraction and ion ratios depend on the formula, including whether extra chemical groups or water of crystallisation are present.
Question 18: A colour change in one qualitative test appears consistent with one possible ion. Does that automatically identify the whole compound? No. Other ions and interfering substances may require additional discriminating evidence under the described procedure.
Question 19: Rewrite “Higher magnesium percentage means this is the safest and most effective garden product.” Answer: “The theoretical magnesium mass percentage differs between the pure compounds. No safety or plant-performance comparison has been established by this calculation.”
Question 20: What would make a scientific claim stronger? Specify a measurable outcome, relevant controlled comparisons, appropriate replicates and conditions, and an analysis that does not overstate the evidence. The question must be designed before collecting observations.
Paper 1 strategy: see the trap inside a short option
Multiple-choice distractors often contain an attractive fragment of truth. An option might correctly identify magnesium as Mg2+ but give MgCl instead of MgCl2. Another might calculate the right numeric formula mass but attach the wrong unit. A third might report the correct percentage while misreading what the percentage describes.
Before choosing an answer, write a compact check: formula → atom count → sum → percentage base. If the question asks about concentration, add volume and ion ratio. This takes less time than rebuilding a muddled calculation after a careless first choice.
Do not choose the longest option simply because it sounds scientific. In a data-based Chemistry question, the correct answer is the one justified by the supplied evidence and chemistry, not the one that makes the most ambitious product claim.
Paper 2 strategy: make your working auditable
Start with the formula and relevant masses. Show the division that produces the fraction and the multiplication by 100 when finding a percentage. For moles, write amount = mass / molar mass. For solutions, write concentration = amount / volume with units. These lines allow an examiner to trace an answer even if a later figure is wrong.
The 2027 K324 structured paper assesses manipulation and interpretation of data as well as recall. Its data-based question may involve information unfamiliar to the student, but the solution can often be built from familiar principles. The right response is to use the given facts and not panic because the setting is new.
Keep the final claim narrower than the working justifies. A perfect calculation of 25.5% does not establish a growing result. Answering an interpretive command requires a sentence that identifies both the computed fact and the scope of the inference.
Practical preparation and scientific integrity
In a practical examination, experimental skills include following instructions, measurements, observations, planning and evaluation. Paper 3 may assess planning and analysis as well as hands-on technique. Use the exact apparatus and safety guidance supplied at school and in the examination, and do not import informal methods from consumer-product demonstrations.
A sound observation record distinguishes what was measured from what was expected. Do not change a result to agree with a textbook or calculation. If a reading is questionable, record that fact, check method and equipment, and explain the limitation when evaluating the investigation.
The official chemistry syllabus emphasises integrity, objectivity and evidence-based decisions. Those are practical habits rather than slogans. This label case trains one of them: refusing to present a theoretical composition as a real-world performance result.
A seven-day progression from basic to advanced
Day one: practise balancing ionic charges for magnesium sulfate and chloride, then extend to other familiar ions. Day two: count atoms and calculate relative formula masses accurately. Day three: convert the sums to elemental percentages, showing the denominator in every calculation.
Day four: reverse the problem from a target amount in moles to a needed mass. Day five: compare same-mass and same-mole samples, stating which quantity was held fixed. Day six: complete solution-concentration exercises with cm3-to-dm3 conversion and the correct ion coefficients.
Day seven: complete an unseen label-claim audit. Include one supported composition statement, one unsupported performance statement and one justified request for more information. Mark errors by type: formula, arithmetic, unit, ratio or inference. Repeat the weakest category instead of copying the same full solution.
Frequently asked questions
Is magnesium sulfate the same as magnesium chloride? No. Both contain magnesium, but their anions, formulae, formula masses and chemical behaviour differ. This teaching case does not recommend substituting one for the other.
Does a higher element mass percentage imply better quality? Not necessarily. It describes a defined proportion in a specified pure compound. Quality and suitability require other information and criteria.
Is a molar mass the same as a relative formula mass? Their numerical values match when expressed using the usual reference scale, but relative formula mass has no units, while molar mass is commonly expressed in g/mol.
What if an exam gives different atomic masses? Use the values supplied and the periodic table printed for that paper. Retain sufficient digits during working and round the final result as instructed.
Do I need to memorise hydrate calculations? This case does not require a special commercial hydrate formula. The syllabus includes formula mass and percentage by mass; follow the actual compound formula and data provided in the question.
Can this be practised at home by mixing substances? The guide provides paper calculations, not experimental directions. Practical investigations belong in properly supervised laboratory settings with suitable safety procedures.
How does this relate to the earlier garden case? The previous volumes taught evidence limits, geometry and responsible public claims. This one supplies a new application of the same discipline: separate an exact symbolic calculation from a conclusion that needs independent observations.
Next reading and official references
Return to Volume 1144 on the EMS garden approval hearing and Volume 1141 on acidity observations to compare the difference between a measured pH and a calculated composition. The Volume 1142 geometry and budget guide shows how constraints change a real-world mathematical conclusion. The earlier limestone Chemistry case provides a second context for reaction evidence. Use the G3 Science route and PSLE Science learning route for broader foundations.
The official source is the 2027 K324 G3 Chemistry syllabus, listed in the SEAB 2027 G3 subject directory, checked on 12 November 2026. This article is a wholly fictional calculation and claim-audit exercise. Always consult SEAB and the candidate’s school for the latest subject details.
