Calculating the number of moles is one of the most essential skills in IB Chemistry. Whether you are working through stoichiometry, titrations, gas laws, energetics, or IAs, mole calculations appear everywhere. Fortunately, the relationship between mass, molar mass, and moles is one of the simplest formulas in the syllabus—once you understand how to apply it correctly and consistently.
This guide gives you a clear, exam-ready explanation that will support you through both SL and HL topics.
Quick Start Checklist
To calculate moles:
moles = mass ÷ molar mass
- Mass is measured in grams.
- Molar mass is in g/mol.
- Moles is the amount of substance in mol.
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Understanding the Formula
The formula comes directly from the definition of molar mass:
Molar mass = mass of substance ÷ number of moles
Rearranging gives:
moles = mass ÷ molar mass
This forms the foundation of nearly all quantitative work in IB Chemistry.
Step-by-Step IB Example
Example:
What is the number of moles in 25 g of sodium chloride (NaCl)?
Molar mass of NaCl = 58.5 g/mol
- Write the formula:
moles = mass ÷ molar mass - Substitute values:
moles = 25 ÷ 58.5 - Calculate:
moles = 0.427 mol (3 significant figures)
This type of calculation is the basis for more complex stoichiometric questions, including limiting reagents and titrations.
Understanding this accuracy is also crucial for Chemistry IA work. To see how expectations differ between subjects, compare the reasoning required in different IB IAs:
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Common IB Exam Variations
IB often tests moles using:
- mass → moles
- moles → mass
- percentage composition → mass → moles
- empirical/molecular formula calculations
- gas laws (n = PV/RT)
- solutions (n = c × V)
In every case, mole calculations are the central step.
Typical Mistakes Students Make
- Using the wrong molar mass (especially with hydrates like CuSO₄·5H₂O).
- Not converting mass to grams.
- Using unrounded values incorrectly.
- Forgetting significant figures.
- Mixing up molar mass and relative atomic mass.
IB examiners award marks for correct working just as much as correct answers, so always show the setup of your calculation.
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Why This Skill Matters
You need mole calculations for:
- Stoichiometry
- Reaction yields
- Limiting reactants
- Empirical and molecular formulas
- Acids, bases, and titrations
- Gas laws
- Redox reactions
- Energetics
- IAs involving quantitative methods
It’s one of the most repeated skills in the entire course.
Building strength in core chemistry reasoning also helps when choosing between the depth of chemistry and the breadth of biology:
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Frequently Asked Questions
1. What if the question gives molar mass in units other than g/mol?
You must convert it to g/mol before using the formula. Moles always use g/mol for mass-based calculations. IB examiners typically expect you to recognize when a unit conversion is necessary.
2. Can you calculate moles without knowing molar mass?
Not from mass alone. You need the molar mass to convert mass into number of particles. However, other formulas—such as n = c × V for solutions or n = PV/RT for gases—can calculate moles using different information.
3. Why does IB Chemistry emphasize mole calculations so much?
Because moles link the microscopic world (atoms, molecules, particles) to macroscopic measurable quantities. Most quantitative chemistry relies on the mole concept, making it essential across the entire course.
Conclusion
Calculating moles from mass and molar mass is one of the most fundamental skills in IB Chemistry. Using the formula moles = mass ÷ molar mass ensures you can approach any stoichiometric calculation confidently, from simple reactions to complex IA data. With RevisionDojo’s chemistry-focused support, you can strengthen the accuracy and clarity needed to excel in all quantitative sections of the course.
