pressure constant ⇒ volume ⬆ to compensate increased collisions (particles).
Avogadro’s Law
According to kinetic theory, the pressure exerted by a gas is due to the number of collisions of its particles per unit area. This, in turn, depends upon the number of particles and their speed.
If you have two containers containing the same amount of a gas at the same temperature and pressure, and you increase the amount of gas in one container. Naturally the pressure of the gas in this container will increase. To keep the pressure constant the volume of the gas must increase. This larger volume compensates for the increased number of particles, ensuring the pressure remains the same. Therefore, at constant temperature and pressure, a greater number of molecules simply requires a larger volume to maintain the same pressure.
These observations led Avogadro to formulate a law which states that equal volumes of different gases must contain an equal number of molecules if the temperature and pressure are kept constant.
Mathematically Avogadro’s law is written as: V ∝ n (at constant temperature and pressure).
(i) How does a gas exert pressure? — Gas particles collide with walls → force per unit area = pressure. More frequent/harder collisions increase pressure.
(ii) How does the volume of a gas change when its temperature is decreased? (Pressure and mass constant) — Temperature ↓ → speed ↓ → collisions less frequent/force ↓. To keep pressure constant, volume ↓ (Charles’ law).
“More moles, more room”
Avogadro: V ∝ n (T,P constant). Think party: more guests need bigger hall.
• V₁/n₁ = V₂/n₂ (T,P fixed)
• Equal volumes (same T,P) contain same # of molecules.
• n = number of moles
Balloon experiment: inflate with 1 breath (small), 2 breaths (bigger) – T, P roughly constant – volume ∝ amount of air (moles).
00-05 min: kinetic theory recap: pressure from collisions.
05-12 min: introduce Avogadro’s law + V ∝ n (constant T,P).
12-20 min: numerical: V₁/n₁ = V₂/n₂ & molar volume concept.
20-30 min: 10 MCQs quiz (solve & discuss).
30-35 min: real-life: breathing, airbags, tyre inflation.
• Avogadro’s law works when T & P are fixed – always check conditions.
• V ∝ n means if moles double, volume doubles (if T,P constant).
• One mole of any gas at STP occupies ~22.4 L – a direct consequence.
• Use ratio V₁/n₁ = V₂/n₂ for problems.
• Attempt quiz without notes first, then check key.