Avogadro's Law
States that equal volumes of all gases at the same temperature and pressure contain an equal number of molecules.
What is Avogadro's Law?
A law stating that equal volumes of all gases, at the same temperature and pressure, contain the same number of molecules.
Key formula / rule: Avogadro's Law (Proportionality)
Key points
- State Avogadro's Law correctly.
- Explain the relationship between gas volume and the number of molecules.
- Apply Avogadro's Law to solve problems involving gases.
- Understand the concept of molar volume.
Common exam trap
Confusing Avogadro's Law with Boyle's Law, Charles's Law, or Gay-Lussac's Law.
Definitions
- Term
Avogadro's Law
- Meaning
A law stating that equal volumes of all gases, at the same temperature and pressure, contain the same number of molecules.
- Term
Molar Volume
- Meaning
The volume occupied by one mole of a substance (typically a gas) at a given temperature and pressure.
- Term
STP (Standard Temperature and Pressure)
- Meaning
A set of conditions defined as 0°C (273.15 K) and 1 atm pressure, used for standardizing gas measurements.
- Term
SATP (Standard Ambient Temperature and Pressure)
- Meaning
A set of conditions defined as 25°C (298.15 K) and 1 bar pressure, used for standardizing gas measurements.
Learning objectives
State Avogadro's Law correctly.
Explain the relationship between gas volume and the number of molecules.
Apply Avogadro's Law to solve problems involving gases.
Understand the concept of molar volume.
Relate Avogadro's Law to stoichiometry.
Formulae
- Name
Avogadro's Law (Proportionality)
- Note
Volume is directly proportional to the number of moles (or molecules) if temperature and pressure are constant.
- Expression
V ∝ n (at constant T and P)
- Name
Avogadro's Law (Ratio Form)
- Note
Used for comparing two states of the same gas or different gases under identical conditions.
- Expression
V1/n1 = V2/n2 (at constant T and P)
- Name
Molar Volume at STP
- Note
Applies to ideal gases at Standard Temperature and Pressure (0°C or 273.15 K and 1 atm).
- Expression
Molar Volume = 22.4 L/mol
- Name
Molar Volume at SATP
- Note
Applies to ideal gases at Standard Ambient Temperature and Pressure (25°C or 298.15 K and 1 bar).
- Expression
Molar Volume = 24.8 L/mol
- Name
Number of Molecules
- Note
NA ≈ 6.022 × 1023 mol⁻¹
- Expression
Number of molecules = Number of moles × Avogadro's number (NA)
Prerequisites
Basic understanding of gases.
Concept of temperature and pressure.
Understanding of molecules and moles.
Proportionality relationships.
Common mistakes
Confusing Avogadro's Law with Boyle's Law, Charles's Law, or Gay-Lussac's Law.
Forgetting to keep temperature and pressure constant when applying the law.
Incorrectly applying the proportionality without considering the conditions.
Using molar volume values without specifying the conditions (STP vs. SATP).
Keywords
Avogadro's Law
Gas Laws
Molar Volume
STP
SATP
Number of Molecules
Moles
Volume
Temperature
Pressure
Stoichiometry
Practice preview
Consider two gases, Hydrogen (H2) and Oxygen (O2), each occupying a volume of 10 litres at 27°C and 1 atm pressure. Which statement is correct regarding the number of molecules in these gases?…
medium
According to Avogadro's Law, equal volumes of all gases at the same temperature and pressure contain:…
easy
Which of the following statements is NOT true regarding Avogadro's Law?…
hard