Physics · Kinetic Theory · NEET
They are the same physical idea, just stated differently. The hypothesis is the word statement: equal volumes of all gases at the same temperature and pressure contain the same number of molecules. Avogadro's law is the mathematical form: at fixed T and P, volume is directly proportional to the number of moles, V proportional to n, or V/n = constant. Use either term in NEET; both point to the same rule.
From the ideal gas equation PV = N kB T, the number of molecules is N = PV/(kB T). If P, V and T are the same for two different gases, then N must be the same. Kinetic theory shows the type of molecule (its mass or size) does not appear in this formula, so only P, V and T decide the count. NCERT states exactly this: if P, V and T are same, then N is also same for all gases.
Avogadro's number (NA = 6.02 x 10^23) is a fixed count, the number of molecules in one mole of any substance. Avogadro's law is a relationship, saying V is proportional to n at fixed T and P. The number is a constant value; the law is a rule connecting volume and moles. They are linked because one mole (NA molecules) of any gas at STP fills the same 22.4 litres.
No. This is the most common trap. Oxygen molecules are heavier than hydrogen molecules, but a 1 litre box of oxygen and a 1 litre box of hydrogen at the same T and P contain the same number of molecules. The heavier gas simply weighs more because each molecule has more mass, not because there are more of them. Mass is more, molecule count is equal.
There are 6.02 x 10^23 molecules, which is exactly one mole. NCERT notes that the number in 22.4 litres of any gas at STP (273 K and 1 atm) is 6.02 x 10^23. Because Avogadro's law says equal volumes hold equal molecules, this 22.4 litre volume (called the molar volume) is the same for every ideal gas at STP.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
At constant temperature and pressure, the volume of a gas is directly proportional to the number of moles: V is proportional to n, so V/n stays constant.
Yes. From PV = nRT, if P and T are held constant then V = (RT/P) n, meaning V is directly proportional to n. Kinetic theory gives the same result through PV = N kB T.
Molar volume is the volume of one mole of gas. At STP (273 K, 1 atm) it is 22.4 litres for any ideal gas, holding 6.02 x 10^23 molecules.
No. It holds for all ideal gases equally. Whether the gas is helium, oxygen or carbon dioxide, equal volumes at the same T and P have equal molecule counts.
It links volume directly to number of molecules, explains the 22.4 litre molar volume, and connects to Dalton's law and the ideal gas equation, which are frequent question sources in Kinetic Theory.
