Stoichiometry errors in IB Chemistry usually come from a small set of recurring problems: using an unbalanced equation, mishandling units, applying the mole ratio incorrectly, overlooking the limiting reactant, or rounding too early. These are not isolated calculation slips. They show that one stage of the stoichiometric method has been skipped or misunderstood.
The most reliable fix is to use the same calculation pathway every time: balance the equation, convert the given quantity to moles, apply the mole ratio, and convert to the requested quantity. Reviewing worked video solutions is especially useful because it reveals where each number comes from and why each step is necessary.
The stoichiometry pathway you should use
Most IB Chemistry stoichiometry questions can be organized into the following sequence:
- Write or identify the balanced chemical equation.
- Convert the information given into an amount in moles.
- Use the coefficients in the equation to apply the mole ratio.
- Convert the resulting amount into the requested mass, volume, concentration, particle number, or percentage.
- Check units, significant figures, physical plausibility, and whether a limiting reactant is involved.
This approach matters because the balanced equation relates amounts of substances in moles, not directly in grams, cubic centimetres, or concentrations. The mole is therefore the bridge between the measured quantity and the chemical ratio.
Useful relationships include:
| Situation | Relationship | Essential unit check |
|---|---|---|
| Mass | n = m / M | Mass in g and molar mass in g mol⁻¹ |
| Solution | n = cV | Volume in dm³ |
| Particles | N = nNₐ | Identify atoms, molecules, ions, or formula units |
| Gas at STP | n = V / Vₘ | Use Vₘ = 22.7 dm³ mol⁻¹ |
| Gas under stated conditions | PV = nRT | Match pressure and volume units to R |
The current IB Chemistry course, first assessed in 2025, places chemical equations and quantitative chemical change within its structure-and-reactivity framework. The official IB Chemistry page provides the current course overview, while the official specimen papers show how calculations appear in the present assessment format.
IB Chemistry stoichiometry common mistakes and their fixes
| Common mistake | Why it causes an error | Practical fix | What to notice in a worked video solution |
|---|---|---|---|
| Using an unbalanced equation | The coefficients do not represent the correct reacting proportions | Balance first and recount every element | The solver establishes the equation before entering values |
| Applying a mole ratio to masses | Coefficients compare moles, not grams | Convert mass to moles before using coefficients | The calculation passes through moles even when the question asks for mass |
| Confusing coefficients and subscripts | Changing a subscript changes the substance itself | Alter coefficients only when balancing | Chemical formulas remain unchanged throughout the solution |
Using cm³ in n = cV | Concentration is normally expressed per dm³ | Divide cm³ by 1000 before substitution | The volume conversion appears before the mole calculation |
| Ignoring the limiting reactant | The wrong reactant is used to predict the product amount | Compare available moles relative to coefficients | Both reactants are tested before product moles are calculated |
| Mixing actual and theoretical yield | Percentage yield is inverted or based on the wrong quantity | Use (actual / theoretical) × 100 | The theoretical quantity comes from stoichiometry first |
| Using 22.7 dm³ mol⁻¹ under any conditions | Molar volume depends on temperature and pressure | Use 22.7 only at STP; otherwise use the stated data or PV = nRT | The solver checks conditions before choosing an equation |
| Rounding intermediate values | Small errors accumulate and may change the final result | Keep unrounded calculator values until the final line | Rounding happens only after the requested quantity is obtained |
Mistake 1: starting the calculation before balancing
Consider the reaction between hydrogen and oxygen. Writing H₂ + O₂ → H₂O and using a 1:1 ratio would be incorrect because the equation is not balanced. The correct equation is:
2H₂ + O₂ → 2H₂O
The mole ratio is therefore 2:1:2. A quick atom count before calculating prevents every later stage from being built on a false ratio.
Never change a subscript to balance an equation. Turning H₂O into H₂O₂ would produce a different compound rather than balance the original reaction.
Mistake 2: skipping the conversion to moles
A coefficient ratio cannot normally be applied directly to masses. In N₂ + 3H₂ → 2NH₃, 1 mol of nitrogen reacts with 3 mol of hydrogen, but their reacting masses are not in a 1:3 ratio because their molar masses differ.
Write units beside every line:
n(N₂) = m(N₂) / M(N₂)
The units g ÷ g mol⁻¹ reduce to mol, confirming that the calculation has reached the quantity required for the mole ratio. The RevisionDojo explanation of the mole is useful if these conversions still feel mechanical rather than meaningful.
Mistake 3: mishandling solution volume
For solutions, n = cV requires volume in dm³ when concentration is in mol dm⁻³. A volume of 25.0 cm³ is 0.0250 dm³, not 25.0 dm³.
A dependable habit is to convert the volume on a separate line before substituting:
25.0 cm³ ÷ 1000 = 0.0250 dm³
Students often know the conversion but forget it under time pressure. In worked solutions, pay attention to when the conversion occurs, not merely to the final numerical answer.
Mistake 4: overlooking the limiting reactant
When quantities of two reactants are supplied, neither should automatically be treated as the limiting reactant. Convert both to moles and compare n ÷ coefficient. The smaller value identifies the reactant that can support less reaction and therefore limits the product.
For example:
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
Suppose 5.00 g of CaCO₃ reacts with 200 cm³ of 0.400 mol dm⁻³ HCl:
n(CaCO₃) = 5.00 / 100.1 = 0.04995 moln(HCl) = 0.400 × 0.200 = 0.0800 mol- Compare
0.04995 / 1with0.0800 / 2 - HCl gives the smaller value, 0.0400, so HCl is limiting
- The equation produces 1 mol CO₂ for every 2 mol HCl, giving 0.0400 mol CO₂
At STP, this corresponds to 0.0400 × 22.7 = 0.908 dm³ of CO₂. The RevisionDojo limiting reactant guide provides further practice with this reasoning.
Mistake 5: confusing yield and purity
Percentage yield compares the amount actually obtained with the maximum amount predicted by stoichiometry:
percentage yield = (actual yield / theoretical yield) × 100
Percentage purity describes how much of an impure sample is the desired substance:
percentage purity = (mass of pure substance / mass of sample) × 100
In a multistep problem, purity may need to be applied before converting to moles, while yield is usually applied after calculating the theoretical product. Read the wording carefully to identify whether the stated percentage changes the reactant available or the product collected.
Mistake 6: using the wrong gas relationship
The current Chemistry data booklet gives the molar volume of an ideal gas at STP as 22.7 dm³ mol⁻¹. This value should not be used automatically when the gas is at a different temperature or pressure.
If non-STP conditions are stated, use PV = nRT unless the question supplies another appropriate relationship. Temperature must be converted to kelvin, and pressure and volume must be compatible with the chosen value of R. The Chemistry data booklet for first assessment 2025 lists the relevant equations and constants.
Mistake 7: treating empirical and molecular formulas as identical
An empirical formula gives the simplest whole-number ratio of atoms. A molecular formula gives the actual number of each type of atom in one molecule.
After converting composition data into moles, divide all values by the smallest. If the ratio is close to 1:1.5, multiply the entire ratio by 2 to obtain 2:3 rather than rounding 1.5 to 2. To find a molecular formula, compare the compound's molar mass with the empirical formula mass and multiply every subscript by the resulting whole-number factor.
Mistake 8: rounding too early or omitting units
Keep several digits in intermediate calculations and round only the final answer. Premature rounding is particularly damaging in limiting-reactant comparisons, empirical-formula ratios, and multistage yield calculations.
A final answer should include an appropriate unit and a sensible number of significant figures based on the data provided. Also perform a plausibility check: a percentage yield should not normally exceed 100%, and the mass of a product should be consistent with conservation of matter once all reactants and products are considered.
How to learn from worked video solutions
Watching a solution passively is not enough. Use RevisionDojo's per-question stoichiometry video solutions as a diagnostic tool:
- Attempt the question independently and preserve your full working.
- Watch only until the first step differs from yours, then pause.
- Label the difference as an equation, conversion, ratio, limiting-reactant, or presentation error.
- Finish the question again without copying the remaining solution.
- Reattempt the same question after one or two days.
- Complete a similar question to test whether the correction transfers to a new context.
Focus on the structure of the reasoning: given quantity → moles → mole ratio → target quantity. A strong video solution also shows unit conversions, explains why a reactant is limiting, and delays rounding until the end.
For targeted practice, use the R2.1.1 chemical equations and stoichiometry Questionbank and the R2.1.2 mole-ratio resources. The wider IB Chemistry resource hub also provides past-paper video walkthroughs, study materials, and exam-style practice. Jojo AI can help explain the first incorrect step, but you should then redo the calculation independently.
A final exam checklist
Before leaving a stoichiometry question, ask:
- Is the chemical equation balanced?
- Did I convert every relevant quantity into moles?
- Did I take the mole ratio from coefficients rather than subscripts?
- If two reactants were given, did I identify the limiting reactant?
- Is the solution volume in dm³?
- Are the gas conditions compatible with the relationship used?
- Did I distinguish theoretical yield, actual yield, and purity?
- Did I keep unrounded intermediate values?
- Does the final answer have a unit and reasonable precision?
Conclusion
The most common stoichiometry mistakes are procedural rather than mysterious. A balanced equation, correct conversion to moles, careful mole-ratio step, and final unit check will prevent most lost marks.
RevisionDojo's Questionbank can provide targeted practice, while its past-paper video solutions show how each calculation should be organized step by step. Use Jojo AI to clarify an error, then reinforce the correction by solving another question without assistance.
Sources and referenced URLs
- International Baccalaureate: Chemistry in the Diploma Programme
- International Baccalaureate: Chemistry specimen papers for first examinations 2025
- Chemistry data booklet for first assessment 2025
- IUPAC Gold Book: stoichiometric number
- RevisionDojo: R2.1.1 chemical equations and stoichiometry videos
- RevisionDojo: R2.1.1 chemical equations and stoichiometry Questionbank
- RevisionDojo: R2.1.2 using the mole ratio
- RevisionDojo: The mole explained clearly
- RevisionDojo: Limiting reagent explained
- RevisionDojo: IB Chemistry resources
