What NEET Asks
- Direct conceptual questions on the proportionality of pressure and temperature.
- Numerical problems involving calculations of pressure or temperature at constant volume.
- Graphical representation and interpretation of Gay-Lussac's Law.
Key Points
- Gay-Lussac's Law states that at constant volume (V) and number of moles (n), the pressure (P) of a fixed mass of gas is directly proportional to its absolute temperature (T).
- Mathematically expressed as P ∝ T or P/T = constant.
- For two different states of a gas at constant volume: P₁/T₁ = P₂/T₂.
- Temperature MUST always be in Kelvin (K) for gas law calculations. K = °C + 273.15.
- The graph of P vs T (in Kelvin) is a straight line passing through the origin.
- The graph of P vs T (in Celsius) is a straight line intersecting the temperature axis at -273.15 °C (absolute zero).
Must-Know Formula / Reaction
- P₁/T₁ = P₂/T₂
- P₁ = Initial pressure of the gas
- T₁ = Initial absolute temperature of the gas (in Kelvin)
- P₂ = Final pressure of the gas
- T₂ = Final absolute temperature of the gas (in Kelvin)
Common Mistakes
- Students often forget to convert Celsius temperature to Kelvin before applying the formula.
- Don't confuse Gay-Lussac's Law with Charles's Law (V-T relationship) or Boyle's Law (P-V relationship).
- Assuming the volume is constant when it is not explicitly stated, leading to incorrect application.
Rapid Revision
Gay-Lussac's Law: P ∝ T (constant V, n). Always use Kelvin (K = °C + 273.15). P₁/T₁ = P₂/T₂. A P-T graph for ideal gases is a straight line through the origin in Kelvin.