Lesson 1 of 4 · Form 3 Chemistry
The gas laws
18 min
Boyle's law
At constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure.
P₁V₁ = P₂V₂
Charles' law
At constant pressure, volume is directly proportional to absolute temperature.
V₁/T₁ = V₂/T₂
Always convert to Kelvin: K = °C + 273.
Combined gas law
P₁V₁/T₁ = P₂V₂/T₂
Example: 200 cm³ of gas at 27°C is heated to 127°C at constant pressure.
V₂ = 200 × 400 ÷ 300 = 266.7 cm³.
Standard conditions
- s.t.p.: 273 K and 760 mmHg (1 atm).
- r.t.p.: about 298 K and 1 atm.
Graham's law of diffusion
The rate of diffusion is inversely proportional to the square root of density (or molar mass). Lighter gases diffuse faster: ammonia (17) diffuses faster than hydrogen chloride (36.5).
Key points to remember
- Boyle: P₁V₁ = P₂V₂
- Charles: V₁/T₁ = V₂/T₂
- Convert °C to Kelvin by adding 273
- Lighter gases diffuse faster
Test yourself
