A7LAM CSCA ACADEMYCSCA Academy / Chemistry / Basic concepts and calculations
CSCA Chemistry · Lesson 4 of 21Gases and the ideal gas equation
Gaz et équation des gaz parfaits · الغازات ومعادلة الغاز المثالي
▶ Watch the video lesson (A7LAM students)
Equal volumes of gases at the same temperature and pressure contain the same number of molecules. That makes gas volumes easy to calculate.
Key ideas
- Avogadro: equal volumes of gases (same T and p) contain equal numbers of moles.
- At STP (0 °C, 101 kPa), 1 mol of any gas occupies 22.4 L.
- Ideal gas equation: pV = nRT (p in Pa, V in m³, T in K).
- Gas density ρ = pM/(RT): heavier gases are denser.
Formulas to know
- V = n × 22.4 L (at STP)
- pV = nRT, R = 8.314 J/(mol·K)
- M = 22.4 × ρ (STP, ρ in g/L)
Worked example
Volume of 8.8 g of CO₂ (M = 44 g/mol) at STP?
- Moles: n = 8.8 / 44 = 0.2 mol
- STP: 1 mol occupies 22.4 L
- Multiply: V = 0.2 × 22.4 = 4.48 L
Answer: 4.48 L
Practice questions
1. Moles in 11.2 L of O₂ at STP?
0.5 mol ✓ · 2 mol · 1 mol · 0.25 mol
11.2 / 22.4 = 0.5.
2. 1 L each of H₂, O₂ and CO₂ at the same T and p. Which has the most molecules?
H₂ · O₂ · CO₂ · All equal ✓
Avogadro's law.
3. A gas has density 1.25 g/L at STP. Molar mass?
28 g/mol ✓ · 14 g/mol · 32 g/mol · 44 g/mol
1.25 × 22.4 = 28.