What stoichiometry formulas must I know for IB Chemistry?

The most important relationships are n = m/M, n = N/NA, and n = cV. Gas questions may also require PV = nRT or a molar gas volume supplied in the question or examination materials. Percentage yield is calculated as actual yield divided by theoretical yield, multiplied by 100. These equations do not replace the balanced chemical equation, because the equation supplies the mole ratio connecting substances. In an exam, write the relevant equation before substituting values so that your method remains clear and easier to check.

How do I know which mole ratio to use?

Take the coefficients directly from the balanced chemical equation and place them in the same order as your conversion. If you are converting from substance A to substance B, multiply by coefficient of B / coefficient of A. For 2H₂ + O₂ → 2H₂O, converting oxygen to water requires multiplication by 2/1, while converting water to oxygen requires 1/2. Include the substance labels in your working until the direction becomes automatic. If your result appears chemically unreasonable, check whether you reversed this ratio.

How can I identify the limiting reactant quickly?

Convert the available amount of each reactant into moles and divide each result by its coefficient in the balanced equation. The reactant producing the smaller normalized value is the limiting reactant. Alternatively, calculate how much product each reactant could form and select the smaller product amount. You cannot reliably identify the limiting reactant from mass alone because substances have different molar masses and equation coefficients. Once identified, use only the limiting reactant to calculate the theoretical yield.

Do I need to show working in IB Chemistry stoichiometry questions?

You should show the balanced equation, mole conversion, ratio, and final conversion whenever the question allows written working. A correct final answer may earn full credit, but visible steps protect method marks when a later arithmetic error occurs. Working also helps distinguish a chemically valid method from a calculator guess. Include units at meaningful stages and avoid rounding intermediate values too aggressively. Multiple-choice questions may not require submitted working, but writing a short mole pathway can still reduce errors.

Why must solution volume be converted from cm³ to dm³?

Concentration in IB Chemistry is commonly expressed in mol dm⁻³, so the volume used in n = cV must be in dm³. Since one dm³ equals 1000 cm³, divide a volume in cm³ by 1000 before substitution. For example, 25.0 cm³ becomes 0.0250 dm³. Forgetting this conversion makes the calculated amount 1000 times too large. An effective exam habit is to write the converted volume beside the original value before starting the calculation.

What is the best way to revise stoichiometry before an IB exam?

Begin by mastering conversions between mass, amount, particles, solution volume, and gas quantities. Next, practise questions in which the required conversion is not explicitly identified, because this develops interpretation rather than formula matching. Keep an error log separating balancing, mole-ratio, unit, limiting-reactant, and arithmetic mistakes. Attempt past or exam-style questions independently before viewing worked solutions, then compare your setup line by line with the demonstrated method. RevisionDojo's topic Questionbank, per-question video walkthroughs, and Jojo AI feedback are most useful when followed by a second attempt without notes.

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IB Chemistry Stoichiometry Explained for Exams

• 5 Aug 2026•12

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IB Chemistry Stoichiometry Explained for Exams

• 5 Aug 2026•12

Stoichiometry is the quantitative use of a balanced chemical equation to calculate amounts of reactants and products. In IB Chemistry, nearly every stoichiometry problem follows one central route: convert the information given into moles, apply the equation's mole ratio, and convert the result into the quantity requested.

This topic is not confined to one type of question. Examiners connect mole ratios to masses, particle numbers, solution concentrations, gas volumes, limiting reactants, percentage yield, atom economy, and experimental data. The aim of this guide is to make that common structure visible so that unfamiliar wording does not disguise a familiar calculation.

Where stoichiometry appears in IB Chemistry

The current IB Chemistry course, first assessed in 2025, organizes content through the themes Structure and Reactivity. The mole is developed under S1.4 Counting particles by mass: the mole, while chemical equations and mole ratios appear in R2.1 How much? The amount of chemical change.

Stoichiometry can appear in Paper 1A multiple-choice questions, Paper 1B data-based and experimental-work questions, and Paper 2 short-answer or extended-response questions. According to the official IB Chemistry subject brief, external examinations contribute 80% of the final grade, so accurate quantitative work matters throughout the course rather than in one isolated unit.

Both SL and HL students need the same fundamental method. HL questions may place that method inside more complex contexts, but the underlying mole logic remains unchanged.

The mole as the bridge between quantities

A balanced equation compares particles, not masses. Because individual particles are impractical to count directly, chemists use the mole as a counting unit. One mole contains exactly 6.02214076 × 10²³ specified entities, according to the NIST definition of the mole.

The entity must be clear. One mole of H₂O means one mole of water molecules, containing two moles of hydrogen atoms and one mole of oxygen atoms.

The essential conversions are:

Information given	Conversion to amount, `n`
Mass, `m`	`n = m / M`
Number of entities, `N`	`n = N / N_A`
Solution concentration, `c`	`n = cV`
Gas volume at stated conditions	Use the given molar volume or gas equation

Here, M is molar mass and N_A is the Avogadro constant. In n = cV, volume must normally be expressed in dm³ when concentration is in mol dm⁻³. The IUPAC definition of amount concentration confirms that concentration is amount of a constituent divided by the volume of the mixture.

The universal stoichiometry method

A reliable solution can be organized into five steps:

Write or identify the balanced equation.
Convert the given quantity into moles.
Apply the stoichiometric mole ratio.
Convert into the quantity requested.
Check units, significant figures, and chemical plausibility.

For example:

2H₂(g) + O₂(g) → 2H₂O(l)

If 3.00 mol of O₂ reacts with excess H₂, the equation gives the ratio:

2 mol H₂O / 1 mol O₂

Therefore:

n(H₂O) = 3.00 × 2/1 = 6.00 mol

The equation coefficients provide a mole ratio, not a mass ratio. This distinction is one of the most important ideas in stoichiometry.

Core calculation types examiners test

Mass-to-mass calculations

Suppose calcium carbonate decomposes as follows:

CaCO₃(s) → CaO(s) + CO₂(g)

Using approximate molar masses, 10.0 g of CaCO₃ is 10.0 / 100.1 = 0.0999 mol. The equation has a 1:1 ratio, so 0.0999 mol of CaO forms. Its mass is therefore 0.0999 × 56.1 = 5.60 g.

A clear response shows both mole conversion and mole ratio. Writing only the final answer risks losing method marks if an arithmetic mistake occurs.

Solution stoichiometry

For a solution, use n = cV. A common trap is substituting a volume in cm³ directly into an equation requiring dm³:

25.0 cm³ = 0.0250 dm³

If 25.0 cm³ of 0.200 mol dm⁻³ NaOH is used, then:

n(NaOH) = 0.200 × 0.0250 = 0.00500 mol

In a titration, the next step is not automatically to assume equal amounts. Apply the coefficients in the balanced neutralization equation. For example, H₂SO₄ and NaOH react in a 1:2 ratio.

Gas stoichiometry

Gas questions may provide a molar gas volume or require use of the ideal gas equation:

PV = nRT

Use consistent units, particularly for pressure, volume, and the selected value of R. Do not apply a memorized molar gas volume without checking the temperature and pressure stated in the question or the values supplied in the examination materials.

For gases at the same temperature and pressure, volume ratios follow equation coefficients. Thus, in N₂ + 3H₂ → 2NH₃, one volume of nitrogen reacts with three volumes of hydrogen to produce two volumes of gaseous ammonia under equivalent conditions.

Limiting reactants

When amounts of two reactants are given, one may be in excess. The limiting reactant is consumed first and determines the maximum amount of product.

For each reactant, calculate either:

the amount of product it could form, or
moles available ÷ stoichiometric coefficient.

The smaller result identifies the limiting reactant. Do not select the reactant with the smaller mass or fewer moles without accounting for the balanced equation.

For N₂ + 3H₂ → 2NH₃, 2.0 mol N₂ would require 6.0 mol H₂. If only 3.0 mol H₂ is available, H₂ is limiting and produces 3.0 × 2/3 = 2.0 mol NH₃.

Percentage yield and atom economy

The theoretical yield is the maximum product predicted from stoichiometry and the limiting reactant. The actual experimental yield is usually smaller.

percentage yield = (actual yield / theoretical yield) × 100

A result above 100% should trigger a check. Possible experimental causes include an impure or wet product, residual solvent, incomplete drying, or measurement error.

Atom economy instead measures how much of the reactant material becomes the desired product:

atom economy = (Mr of desired product × its coefficient / total Mr of reactants × coefficients) × 100

Percentage yield describes experimental success, whereas atom economy evaluates the reaction pathway. They are not interchangeable.

Empirical and molecular formulas

For an empirical formula question:

Convert each element's mass or percentage into moles.
Divide all mole values by the smallest.
Convert the ratios into the smallest whole numbers.

If ratios are close to 1:1.5, multiply both by 2 rather than rounding 1.5 to 2. To obtain a molecular formula, divide the compound's molar mass by the empirical formula mass, then multiply every empirical subscript by that integer.

How IB questions phrase stoichiometry tasks

Recognizing command language helps you select the required response:

Wording	What you should do
Calculate	Show numerical working and units
Determine	Reach a conclusion from calculation or data
Deduce	Use preceding information to infer the answer
Show that	Demonstrate how the stated result follows
Identify the limiting reactant	Compare reactants using the equation ratio
Hence calculate	Use the result from the previous part

A question may not use the word “stoichiometry.” Instead, it may ask for the mass produced, concentration of an unknown solution, volume of gas released, excess reactant remaining, or empirical formula of a compound. Translate the wording into the mole pathway before entering numbers into a calculator.

Common mistakes that lose marks

The most frequent errors are procedural rather than conceptual:

using an unbalanced equation
treating coefficients as mass ratios
reversing the required mole ratio
failing to convert cm³ to dm³
calculating from an excess reactant instead of the limiting reactant
using the mass of a solution as though it were the mass of solute
rounding intermediate values too early
omitting units or giving inappropriate significant figures
reporting a numerical answer without enough working

Keep extra digits during the calculation and round only the final result. Match the precision justified by the supplied data unless the question gives a specific instruction.

An exam-focused practice strategy

First, learn the conversion equations well enough to identify them from units. Then practise mixed questions so that you must decide which conversion applies rather than being told.

A useful revision sequence is:

Review counting particles by mass and the mole.
Consolidate chemical equations and stoichiometry.
Practise using the mole ratio.
Attempt mixed problems in the stoichiometric relationships Questionbank.
Move to timed practice through the IB Chemistry resource hub.

After each question, classify the error as equation balancing, conversion, ratio, arithmetic, units, or interpretation. This diagnosis is more useful than simply recording that the answer was wrong.

To see how the method converts into marks, use RevisionDojo's stoichiometry worked video solutions and per-question video walkthroughs available through the Chemistry past-paper area. Watching a solution is most effective after attempting the question independently, because you can compare each line of your method with the worked approach.

Conclusion

IB Chemistry stoichiometry centres on a small number of repeatable ideas: convert to moles, use a balanced equation, apply the correct mole ratio, and convert to the requested quantity. Limiting reactants, gas calculations, titrations, yields, and empirical formulas are applications of that same structure.

For revision, combine careful written working with varied exam-style practice. RevisionDojo's Chemistry Questionbank, Jojo AI feedback, worked stoichiometry videos, and Chemistry predicted papers can help you move from knowing formulas to applying them under examination conditions.

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Where stoichiometry appears in IB Chemistry

Both SL and HL students need the same fundamental method. HL questions may place that method inside more complex contexts, but the underlying mole logic remains unchanged.