What Is the Combined Gas Law?
The combined gas law merges Boyle's Law, Charles's Law, and Gay-Lussac's Law into a single equation that describes the relationship between pressure, volume, and temperature of a fixed amount of gas:
(P₁ × V₁) / T₁ = (P₂ × V₂) / T₂
Where P = pressure, V = volume, T = temperature (in Kelvin)
Subscript 1 = initial conditions, 2 = final conditions
Component Laws
Boyle's Law: At constant temperature, pressure and volume are inversely proportional (P₁V₁ = P₂V₂).
Charles's Law: At constant pressure, volume and temperature are directly proportional (V₁/T₁ = V₂/T₂).
Gay-Lussac's Law: At constant volume, pressure and temperature are directly proportional (P₁/T₁ = P₂/T₂).
Why Kelvin?
Temperature must be in Kelvin because gas laws require an absolute temperature scale. To convert: K = °C + 273.15. Using Celsius or Fahrenheit directly in gas law calculations gives incorrect results.
Applications
The combined gas law applies to weather balloons, tire pressure changes, scuba diving, HVAC systems, and any scenario where a gas undergoes changes in pressure, volume, or temperature simultaneously.
How to Use
Enter the initial pressure, volume, and temperature, plus two of the three final values. The calculator solves for the missing final value. Select which variable to solve for.
Limitations
The combined gas law assumes an ideal gas. Real gases deviate from ideal behavior at high pressures, low temperatures, or when intermolecular forces are significant.