IGCSE Moles and Stoichiometry Made Simple: The Complete Notes

August 1, 2026 by Trevelyan

Nowa Link Notes

IGCSE Moles and Stoichiometry Made Simple: The Complete Notes

These IGCSE moles and stoichiometry notes exist because most students meet the mole concept as a pile of separate formulae: one for mass, one for gas volume, one for concentration, another for empirical formula, and try to memorise all of them at once. That’s usually where the confusion starts. Under exam pressure, five or six unconnected formulae are hard to recall accurately, and it’s easy to reach for the wrong one.

The mole concept in Cambridge IGCSE Chemistry (0620) is actually one idea, applied in different situations. Once that idea is clear, stoichiometry stops being a memory test and becomes a repeatable process. These IGCSE moles and stoichiometry notes build it in order, the way it’s meant to be understood: as a ladder, where each step depends on the one below it.

Step 1: Ar and Mr, turning a formula into a mass

Relative atomic mass (Ar) comes straight from the periodic table: it’s the larger of the two numbers next to each element’s symbol, not the smaller one (the smaller number is the atomic number, and mixing the two up is one of the most common early errors in this topic).

Relative formula mass (Mr) is just the Ar values added together according to the formula. For sodium carbonate, Na₂CO₃:

Mr = (2 × 23) + 12 + (3 × 16) = 106

This step feels like simple arithmetic, but it’s actually the bridge between a chemical formula and a real, measurable mass in grams. Get it wrong here, and every calculation built on top of it is wrong too.

Step 2: what a mole actually is

A mole is a counting unit: the same idea as “a dozen,” except the number is enormous, because atoms are enormously small. One mole of any substance contains 6.02 × 10²³ particles (the Avogadro constant), and the mass of one mole, in grams, equals its Ar or Mr.

This is the conceptual leap worth pausing on. Students who treat “mole” as an abstract exam word rather than an actual quantity of substance tend to struggle with everything that follows, and it’s rarely because the maths is hard; usually it’s because they’re manipulating a symbol they don’t really picture.

Step 3: the three routes into moles

That’s the core of these IGCSE moles and stoichiometry notes: every stoichiometry question, underneath all the different wording, is about getting to moles and then getting back out again. There are exactly three ways in:

From mass: moles = mass (g) ÷ molar mass (g/mol)
From gas volume at r.t.p.: moles = volume (dm³) ÷ 24
From solution concentration: moles = concentration (mol/dm³) × volume (dm³)

Each of these also runs in reverse: mass = moles × Mr, volume = moles × 24, concentration = moles ÷ volume. Learning them as three relationships with an inverse each, rather than six separate formulae, cuts the memory load in half.

One detail that trips students up specifically in the gas and concentration routes: volumes are often given in cm³ but the formulae need dm³. Divide by 1000 to convert cm³ to dm³ before using either formula, and do it as the first step rather than an afterthought at the end.

Step 4: the balanced equation as a ratio machine

This is the step that turns “moles” into “stoichiometry.” The coefficients in a balanced equation give the mole ratio between reactants and products. A student who’s solid on Steps 1–3 but skips this one will correctly find the moles of the substance they’re given, then quietly assume a 1:1 ratio for the substance they actually need, which is the single most common source of lost marks in this whole topic.

The habit that fixes it: read the ratio off the equation explicitly, in writing, before calculating anything. Not mentally. Written down, every time.

Step 5: applying the ladder

This is where these IGCSE moles and stoichiometry notes turn into exam technique. Once Steps 1–4 are solid, everything else in stoichiometry is the same four-move sequence (equation, moles of what you know, ratio, moles/mass/volume of what you need) applied to a specific situation:

Limiting reactant: calculate moles of each reactant given, compare each to what the equation requires, and identify which one runs out first. The reactant left over doesn’t affect how much product forms.
Percentage yield: (actual yield ÷ theoretical yield) × 100. The theoretical yield has to come from a full stoichiometric calculation based on the limiting reactant — never a guess.
Percentage composition or purity: mass of the pure component ÷ total mass, ×100.
Empirical and molecular formula: convert masses (or percentages) to moles, divide every value by the smallest number of moles to get a ratio, then convert that ratio to whole numbers.
Titrations: use the volume and concentration of the solution you know to find its moles, apply the equation’s ratio, then find the moles, concentration, or volume of the second solution.

Seen this way, a student isn’t learning eight unrelated formulae; they’re applying one process to eight different contexts, which is the whole point of building these IGCSE moles and stoichiometry notes as a ladder rather than a list.

Multi-step questions are just two familiar steps, joined

The questions that feel hardest, mass of one reactant to mass of a completely different product, aren’t a new type of maths, just a longer one. There’s no single formula that jumps straight from one to the other: mass → moles (Step 3), moles → moles (Step 4, the ratio bridge), moles → mass (Step 3 again, for the new substance). Once a student sees the bridge in the middle, “hard” multi-step problems stop feeling unfamiliar.

Complete formula reference

The table below is the compressed version of these IGCSE moles and stoichiometry notes: every formula in one place, plus the exact spot students lose marks.

Find this	Formula	Watch for
Relative formula mass, Mr	Sum of (Ar × number of each atom)	Use Ar, not atomic number
Moles from mass	moles = mass (g) ÷ Mr	Mass always in grams
Mass from moles	mass = moles × Mr
Moles from gas volume	moles = volume (dm³) ÷ 24	r.t.p. only; convert cm³ → dm³ by ÷1000
Moles from concentration	moles = concentration (mol/dm³) × volume (dm³)	Convert cm³ → dm³ by ÷1000
Mole ratio between substances	Read from the balanced equation’s coefficients	Never assume 1:1
Percentage yield	(actual yield ÷ theoretical yield) × 100	Theoretical yield needs a full calculation
Empirical formula ratio	moles of each element ÷ smallest moles value	Never round — multiply to clear fractions

The takeaway from these IGCSE moles and stoichiometry notes

Stoichiometry looks like a long list of formulae from the outside. From the inside, it’s one idea, moles as a counting unit, applied through three ways in, one ratio step, and a handful of applied skills built on top. Students who learn it as a connected ladder, rather than a list to memorise, tend to find that the “harder” topics later in the syllabus (electrolysis calculations, energetics, titrations) get noticeably easier too, because they’re reusing the same toolkit.

If your child understands each piece on its own but struggles to connect them under exam conditions, that’s a completely normal stage, and it’s exactly what we work through in Nowa Link’s IGCSE moles and stoichiometry coaching: building the connections between the steps until the whole ladder feels like one process instead of eight, rather than re-explaining the mole concept from scratch.

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The Mole Formula Explained: A Step-by-Step IGCSE Guide

August 1, 2026 by Trevelyan

Nowa Link Notes

The Mole Formula Explained: A Step-by-Step IGCSE Guide

Ask most IGCSE Chemistry students for the IGCSE moles formula and they’ll recite something correctly, then freeze the moment a question doesn’t look exactly like the one they memorised it from. That’s rarely a knowledge gap; more often it’s a sign the formula was learned as a string of symbols, not as something with a reason behind it.

This guide walks through where the IGCSE moles formula actually comes from, then works one full question start to finish, so the process, not just the equation, is the thing that sticks.

Where the IGCSE moles formula comes from

A mole is a counting unit. One mole of any substance, whatever it is, contains 6.02 × 10²³ particles: the Avogadro constant. That number is fixed, but the mass of one mole is different for every substance, because it depends on how heavy each particle is.

The mass of one mole of a substance, in grams, is equal to its relative formula mass, Mr. So if you know how many grams of a substance you have, and you know its Mr, you can work out how many “counting units” (moles) that mass represents:

moles = mass (g) ÷ Mr

That’s the whole idea. Everything else is the same relationship, rearranged or adapted for a different starting point.

The three versions of the IGCSE moles formula

Cambridge IGCSE questions give you a mole calculation from one of three starting points, and each has its own version of the formula:

1. From mass: moles = mass (g) ÷ Mr (or molar mass)
2. From gas volume at room temperature and pressure (r.t.p.): moles = volume (dm³) ÷ 24
3. From solution concentration: moles = concentration (mol/dm³) × volume (dm³)

Each one also works in reverse: rearrange to find mass, volume, or concentration instead of moles. Students who learn these as three separate, unrelated formulae tend to guess under pressure. Students who understand that all three are just different doors into the same idea, “how many counting units do I have,” tend to pick the right one automatically, because they’re thinking about what information the question gives them rather than searching their memory for a matching formula.

A full worked example

Question: Calculate the number of moles in 8.5 g of ammonia, NH₃. (Ar: N = 14, H = 1)

Step 1: find Mr. Mr of NH₃ = 14 + (3 × 1) = 17

Step 2: identify which version of the formula applies. The question gives a mass in grams, so this is the “from mass” version: moles = mass ÷ Mr.

Step 3: substitute the numbers. moles = 8.5 ÷ 17 = 0.5

Step 4: state the answer with its unit. moles of NH₃ = 0.5 mol

Four steps: find Mr (if needed), identify which formula the question is actually asking for, substitute, and state the answer with a unit. That last step matters more than it looks: a correct number with no unit, or the wrong unit, can lose the mark even when the working is right.

Where students lose marks even when they know the formula

The most common mistake is reading the atomic number instead of Ar: Ar is the larger number next to an element’s symbol on the periodic table, not the smaller one, and this mix-up is common early in the topic and quietly wrong from Step 1 onward. A close second is forgetting to convert cm³ to dm³, since the gas volume and concentration formulae both need dm³; if a question gives a volume in cm³, divide by 1000 before substituting into the formula, not after. Rounding too early is another one worth watching, so carry full calculator precision through every intermediate step of a multi-step question and round only the final answer, to the precision the question asks for. And leaving out the unit costs marks even when the number itself is right: “0.5” and “0.5 mol” are not the same answer on a mark scheme.

Practise the IGCSE moles formula, not just the recall

The formula itself is short enough to memorise in five minutes. What actually takes practice is recognising which version a question is asking for, and working through it cleanly under time pressure, which is a different skill from knowing the equation exists.

If your child can recite the IGCSE moles formula but freezes when a question phrases it differently than their notes do, that’s a completely normal gap, and it’s exactly the kind of thing we work on directly in Nowa Link’s IGCSE chemistry coaching: rather than re-teaching the formula, we focus on building the pattern-recognition that makes it usable under real exam conditions.

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Mastering Mole Calculations for IGCSE Chemistry

August 1, 2026 by Trevelyan

Nowa Link Notes

Mastering Mole Calculations for IGCSE Chemistry

There’s a specific kind of student who can recite every formula for IGCSE mole calculations correctly, explain the mole concept clearly when asked directly, and still lose marks in the actual exam. It’s a confusing place to be, because it doesn’t look like a knowledge gap from the outside; the student clearly knows the material. What’s usually missing is fluency: the ability to recognise which formula a question needs, without conscious searching, while also tracking three or four other things at once.

Mastering IGCSE mole calculations comes down to how those formulae get practised, not how many more of them a student learns.

Know the formula vs. use the formula

These are different skills, and IGCSE Chemistry tests the second one almost exclusively. A student can know, in isolation, that moles = mass ÷ Mr, and still freeze when that formula shows up as step two of a four-part question about limiting reactants. The gap between “knows it” and “uses it automatically inside a bigger problem” is the actual work of mastering this topic, and it’s closed by a specific kind of practice, not by re-reading notes.

The three-step discipline behind IGCSE mole calculations

One habit shows up again and again as the thing that separates students who lose marks from students who don’t: write the formula, substitute the numbers, state the answer with its unit. Not mentally. Visibly, every time, even for questions that feel too easy to need it.

This matters for two reasons. First, it’s where method marks come from: a large share of the marks in stoichiometry go to showing the process, on top of the final number itself. Second, it’s a habit that holds up under exam pressure precisely because it doesn’t depend on staying calm. A rushed, anxious student who’s trained to always write the formula first still gets the structure right, even if the arithmetic wobbles.

The formula triangle: useful, but not the whole story

Many students are taught the formula triangle (mass at the top, moles and Mr at the bottom corners; cover what you want to find, and the triangle shows whether to multiply or divide). It’s a genuinely useful tool, especially for students who freeze up rearranging equations under time pressure.

It’s worth knowing both the triangle and the plain formula, though, rather than relying on the triangle alone. A triangle memorised without understanding why it works can become a crutch — reliable until the moment a student can’t quite remember which corner goes where, at which point there’s nothing to fall back on. Understanding the underlying relationship (moles = mass ÷ Mr, and its reverse, mass = moles × Mr) means the triangle becomes a shortcut on top of real understanding, not a replacement for it.

Multi-step problems are two familiar steps, joined

This is where multi-step IGCSE mole calculations start to feel unfamiliar: a question that goes from “mass of one reactant” to “mass of a completely different product” doesn’t have a single formula that jumps straight from one to the other. But it isn’t a new type of question, just a longer one: mass → moles (a formula you already know), moles → moles (the equation’s ratio connecting the two substances), then moles → mass again for the new substance (the same formula, used a second time).

Framing it this way, as two known steps stitched together by one ratio, is often the single biggest unlock for students who find multi-step questions disproportionately harder than single-step ones. The maths isn’t new; the structure just needs to be seen once, explicitly, before it stops feeling foreign.

Deliberate variation beats repetition

Doing ten different question styles on the same underlying concept builds more real competence than redoing the same style ten times. The failure mode to watch for is a student who’s fluent with “the mass-to-mass question” specifically, but falls apart the moment a gas volume or a concentration gets introduced into the same multi-step problem, because they’ve practised one question shape rather than the skill underneath it.

Good practice for IGCSE mole calculations deliberately mixes the three routes into moles (mass, gas volume, concentration), forces students to identify which one applies each time, and occasionally strings two or three together in one question, which is exactly how the real exam is structured.

Trace errors back to the step that broke

When a calculation comes out wrong, the productive response is tracing back through the working to find exactly which step introduced the error, rather than checking the final answer against a mark scheme and moving on. Wrong Mr? Wrong ratio? Unconverted unit? That specific diagnosis is what turns a mistake into something that gets fixed permanently, rather than one that quietly resurfaces in a different question a week later.

What mastery of IGCSE mole calculations looks like

A student who has mastered mole calculations isn’t necessarily faster at the arithmetic than anyone else. They’re faster at recognising, almost instantly, which formula a question is asking for — because they’ve practised enough variety that the pattern-matching has become automatic, and they’ve built the habit of writing the formula down every time, so the method is never in question even when the numbers get tricky.

That kind of fluency comes from practice, not memorisation. It’s exactly what we focus on in Nowa Link’s IGCSE mole calculations coaching: building the recognition and habits that make already-known formulae usable under real exam conditions, rather than reteaching them from scratch.

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IGCSE Stoichiometry Notes: Everything You Need Before the Exam

July 31, 2026 by Trevelyan

IGCSE Stoichiometry: Why You Keep Losing Marks on Questions You Actually Know

This is a condensed run-through for the day or two before the exam. You already understand stoichiometry, the goal here is making sure nothing on this list has quietly gone soft. Read it end to end once, then use it as a checklist against your own working on a few practice questions.

The full syllabus scope, in one place

Cambridge IGCSE Chemistry 0620 (Extended tier) examines the following under stoichiometry:

Stating formulae of elements and compounds, and constructing word and symbol equations with state symbols
Deducing formulae of ionic compounds from charges, and constructing ionic equations
Relative atomic mass (Ar) and relative formula mass (Mr)
Calculating reacting masses in simple proportions
The mole as a unit of amount of substance, and the Avogadro constant (6.02 × 10²³)
moles = mass ÷ molar mass, and using it to find mass, molar mass, or number of particles
The molar gas volume: 24 dm³ at r.t.p.
Stoichiometric calculations involving reacting masses, limiting reactants, gas volumes, and solution concentrations (g/dm³ and mol/dm³)
Titration calculations
Empirical and molecular formula from experimental data
Percentage yield, percentage composition, and percentage purity

If any item on this list feels unfamiliar rather than just “a bit rusty,” that’s worth flagging now, not discovering mid-exam.

Every formula in one table

Find this	Formula	Watch for
Relative formula mass, Mr	Sum of (Ar × number of each atom)	Use Ar, not atomic number
Moles from mass	moles = mass (g) ÷ Mr	Mass always in grams
Mass from moles	mass = moles × Mr
Moles from gas volume	moles = volume (dm³) ÷ 24	r.t.p. only; convert cm³ → dm³ by ÷1000
Gas volume from moles	volume (dm³) = moles × 24	Convert to cm³ by ×1000 if needed
Moles from concentration	moles = concentration (mol/dm³) × volume (dm³)	Convert cm³ → dm³ by ÷1000
Concentration from moles	concentration = moles ÷ volume (dm³)
Mole ratio between substances	Read from the balanced equation’s coefficients	Never assume 1:1
Percentage yield	(actual yield ÷ theoretical yield) × 100	Theoretical yield needs a full calculation
Percentage composition/purity	(mass of component ÷ total mass) × 100
Empirical formula ratio	moles of each element ÷ smallest moles value	Never round; multiply to clear fractions
Number of particles	moles × 6.02 × 10²³	Avogadro constant

Command words to recognise on sight

Calculate: a numerical process is required, shown clearly, not just a final number.
Determine: often an unstructured, multi-step problem; you have to plan the route yourself.
State: precise wording, no working needed, but exact phrasing matters (“mol” is accepted, “moles” as a plural is not, for example).
Show that: the answer is given to you; marks come from the method, not from confirming the number works. Full working is non-negotiable here.
Deduce: conclude from the information given, common in empirical formula and ionic equation questions.

The five checks to run before submitting any stoichiometry answer

1. Did you use Ar (the larger periodic table number), not the atomic number?
2. Did you convert every volume to dm³ before substituting into a formula?
3. Did you read the mole ratio off the balanced equation, rather than assuming 1:1?
4. If a mole ratio wasn’t a whole number, did you multiply to clear the fraction, rather than round?
5. Is your working visible, and does your final answer include the correct unit?

Running through these five checks on a few past questions tonight is a better use of time than doing ten more fresh questions. It targets exactly the places marks are lost, rather than practising what’s already solid.

One last thing worth remembering

Stoichiometry marks are disproportionately method marks. A wrong answer with clear, structured working often scores more than a right answer with none. If time is short, prioritise writing every step down over chasing speed. A slower, fully-shown answer beats a fast, unshown one almost every time this topic is assessed.

If any part of this list still feels shaky the night before, that’s still fixable. It’s exactly the kind of targeted, last-mile gap we help close in Nowa Link’s IGCSE chemistry coaching.

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5 Stoichiometry Mistakes That Cost IGCSE Chemistry Students Marks

July 29, 2026July 27, 2026 by Trevelyan

Nowa Link Notes

5 Stoichiometry Mistakes That Cost IGCSE Chemistry Students Marks

Most students who struggle with stoichiometry in Cambridge IGCSE Chemistry don’t struggle because they can’t calculate. They can. Given a quiet room and no time pressure, most of them can work through a moles question just fine.

They lose marks in the actual IGCSE Chemistry exam for a smaller, more specific reason: five predictable slips that show up in Cambridge’s own examiner reports, series after series. None of them are about ability. All of them are fixable in an afternoon, once a student knows to look for them.

This matters more than it might seem, because stoichiometry isn’t really “one topic” in IGCSE Chemistry 0620. It’s the mathematical spine of the whole Extended syllabus. Electrolysis calculations, energetics, titrations, and organic yield questions are all the same four moves wearing different clothes. Get the moles logic solid once, and a large share of the numerical exam gets easier at the same time.

The IGCSE Chemistry slip that costs the most marks: assuming a 1:1 ratio

This is the single highest-frequency error in stoichiometry, and it’s almost never about the maths. A student correctly finds the moles of the substance they’re given, then carries that number straight across to the substance they need to find, without checking the balanced equation’s coefficients. A 2:1 or 3:2 ratio gets silently treated as 1:1.

The fix is a habit, not a formula: read the ratio off the equation explicitly, in writing, before calculating anything. Not mentally. Written down.

Confusing dm³ and cm³ at the very last step

Cambridge’s June 2024 examiner report describes candidates who correctly found the moles of one reactant, correctly applied the equation’s ratio to find the moles of the next substance, genuinely strong work, and then lost the mark on the final line by multiplying by 24 instead of 24,000, giving an answer in dm³ when the question asked for cm³.

This is worth sitting with, because it isn’t a conceptual gap. The student understood the chemistry. What was missing was one habit: checking what unit the final answer needs to be in before doing the last multiplication, not after.

Rounding an empirical formula ratio instead of clearing it

When a mole ratio comes out as something like 1 : 2.5, the correct move is to multiply every part of the ratio by whatever number clears the fraction: ×2 for .5, ×3 for .33, ×4 for .25. Examiner reports show candidates instead rounding 2.5 down to 2, turning a correct ratio into an incorrect formula.

A related version of the same mistake: calculating a ratio of 5, then writing the answer as “(CH₂)₅” instead of expanding it to C₅H₁₀. A molecular formula has to be written out in full, the shorthand isn’t accepted, even when the underlying number is right.

Skipping the equation on percentage questions

For percentage yield and percentage composition questions, examiners note that weaker candidates often “made no attempt to use the equation or the moles of reactants and products”, in other words, they tried to shortcut a question that required the full mole-ratio method. This looks like a knowledge gap. It’s usually a confidence gap: the student recognised the question as “the hard kind” and disengaged from method rather than working through it.

The percentage itself is simple arithmetic. The theoretical yield it depends on isn’t a guess, it has to come from a proper stoichiometric calculation based on the limiting reactant.

Not showing the working

A large share of the marks in this topic are method marks, not answer marks. On “Calculate” and “Show that” questions especially, a numerically correct answer with no visible working can lose marks it would otherwise earn, and a wrong answer with clearly structured working often picks up partial credit it wouldn’t get if left blank. In stoichiometry, the working is the evidence, not just a formality.

The four-step process underneath all of it

Once these five slips are named, the actual method is short. Every IGCSE Chemistry stoichiometry question, from a straightforward reacting-mass calculation to a multi-step titration, runs on the same sequence:

Get to moles. From mass (moles = mass ÷ Mr), from gas volume at r.t.p. (moles = volume in dm³ ÷ 24), or from concentration (moles = concentration × volume).
Read the ratio. Off the balanced equation’s coefficients, written down, never assumed.
Apply the ratio to get the moles of whatever the question is actually asking for.
Convert back out to mass, volume, or concentration, whichever the question needs, in the unit it needs.

Multi-step questions that feel unfamiliar, e.g. mass of one reactant to mass of a completely different product aren’t a new type of question. They’re this same sequence run twice, joined by the ratio step in the middle. Once a student sees that, “hard” multi-step problems stop feeling like a different subject.

Quick-reference formula sheet

Find this	Formula	Watch for
Relative formula mass, Mr	Sum of (Ar × number of each atom)	Use Ar, not atomic number
Moles from mass	moles = mass (g) ÷ Mr	Mass in grams
Moles from gas volume	moles = volume (dm³) ÷ 24	r.t.p. only; ÷1000 to convert cm³ → dm³
Moles from concentration	moles = concentration (mol/dm³) × volume (dm³)	÷1000 to convert cm³ → dm³
Percentage yield	(actual yield ÷ theoretical yield) × 100	Theoretical yield needs a full stoichiometric calculation
Empirical formula ratio	moles of each element ÷ smallest moles value	Never round, multiply to clear fractions
Mole ratio between substances	Read from the balanced equation’s coefficients	Never assume 1:1

The takeaway

None of this is about a student being “bad at chemistry.” Cambridge’s own examiner reports show the same five slips recurring across series, in candidates who otherwise understand the method. The gap is precision and process, not raw ability, which is also why it’s one of the more coachable parts of the IGCSE exam.

If your child is doing well on the concepts but the marks aren’t showing it, that’s usually where the problem actually is. It’s also exactly what we work on in our weekly IGCSE chemistry class, not re-teaching the mole concept from scratch, but finding the specific step where marks are quietly leaking, and closing it.

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