Avogadro’s Law is one of the foundational gas laws in IB Chemistry, forming part of the broader ideal gas relationship used across stoichiometry, experimental design, and HL gas behavior questions. Despite its simplicity, many students struggle to apply the law correctly in calculations or overlook the conditions under which it applies. This guide explains the law clearly and provides IB-style reasoning to help you earn full marks.
Quick Start Checklist
Avogadro’s Law states that:
- Equal volumes of gases
- At the same temperature and pressure
- Contain the same number of moles
Mathematically:
V ∝ n (volume is directly proportional to moles)
Strengthening your practical and conceptual chemistry foundations makes these gas relationships far easier to apply:
https://www.revisiondojo.com/blog/tips-to-improve-your-lab-skills-for-ib-chemistry
IB Definition of Avogadro’s Law
The most precise definition used in IB Chemistry is:
“At the same temperature and pressure, equal volumes of gases contain equal numbers of particles.”
This relationship applies to all gases, whether they are monatomic or polyatomic, as long as temperature and pressure are constant.
Mathematical Expression
Avogadro’s Law can be written as:
- V₁ / n₁ = V₂ / n₂
- V ∝ n
- n = V / molar volume (for gases at STP)
At standard temperature and pressure (STP):
- Molar volume = 22.7 dm³ mol⁻¹ (IB standard)
This value is essential for stoichiometry involving gases.
Why Avogadro’s Law Works
1. Gases are mostly empty space
Gas particles are far apart relative to their size.
The identity of the gas does not significantly affect how much space it occupies under the same conditions.
2. Particle number determines volume
At fixed temperature and pressure, gas volume depends on the number of particles, not:
- particle mass
- molecular structure
- density
This makes gases uniquely predictable.
This type of conceptual clarity is essential for designing and interpreting Chemistry IA experiments, especially those using gas collection or syringe measurements:
https://www.revisiondojo.com/blog/ib-biology-ia-vs-chemistry-ia-which-is-more-challenging
Common Applications in IB Chemistry
1. Gas stoichiometry
You can use volume ratios directly instead of converting to moles:
Example:
N₂(g) + 3H₂(g) → 2NH₃(g)
Volume ratio = 1 : 3 : 2
2. Determining molar volume
If you measure gas volume and know moles, you can determine molar volume experimentally.
3. Ideal gas law integration
Avogadro’s Law is built into PV = nRT, where:
- more moles (n) → more volume (V)
4. Balancing equations
Sometimes gas volume data help determine mole ratios and reaction coefficients.
Understanding gas behavior also helps compare the depth of IB Chemistry with the broader environmental focus of ESS:
https://www.revisiondojo.com/blog/ib-ess-vs-ib-chemistry-which-is-more-useful-for-environmental-careers
IB Exam Tips
- Always check if temperature and pressure are constant before applying V ∝ n.
- Do not assume all gases have the same density. Avogadro’s Law applies to moles, not mass.
- Use 22.7 dm³ mol⁻¹ only at STP as defined by the IB.
- For non-STP conditions, use PV = nRT instead.
For broader science subject planning—including whether chemistry or biology better matches your goals—see:
https://www.revisiondojo.com/blog/ib-biology-vs-ib-chemistry-which-science-is-better-for-you
Frequently Asked Questions
1. Does Avogadro’s Law apply to real gases?
It applies approximately. Real gases deviate slightly at high pressures and low temperatures. However, under standard conditions, they behave close enough to ideal gases that the law works well for calculations.
2. Why is molar volume always 22.7 dm³ mol⁻¹ in IB?
This value comes from the ideal gas law at standard temperature (273 K) and standard pressure (100 kPa), which the IB defines as STP. Some textbooks use 22.4, but IB uses 22.7 consistently.
3. Can Avogadro’s Law be used for liquids or solids?
No. Liquids and solids have fixed densities and do not change volume in proportion to particle number. The law applies only to gases because their particles are far apart and compressible.
Conclusion
Avogadro’s Law shows that gas volume is directly proportional to the number of moles at constant temperature and pressure. It provides one of the simplest yet most powerful tools for gas calculations in IB Chemistry, allowing students to predict gas behavior, perform stoichiometry, and integrate gas laws. With RevisionDojo’s chemistry-focused approach, you can build the clarity and confidence needed to master this key topic.
