Physics · Kinetic Theory · NEET
For NEET physics (NCERT Kinetic Theory), STP uses 0 C = 273.15 K (usually taken as 273 K). The 25 C value belongs to newer chemistry/thermodynamics conventions, not the NCERT physics STP. In this chapter always use 273 K unless the question says otherwise.
Put n=1 mole into PV = nRT. V = nRT/P = (1 x 8.31 x 273) / (1.01x10^5). That gives V = 2268 / 101000 = 0.0224 m^3 = 22.4 litres. It is not a magic number, it just falls out of the ideal gas equation at 273 K and 1 atm.
It holds for ANY ideal gas: helium, oxygen, hydrogen, nitrogen, all the same. This is Avogadro's hypothesis: equal volumes of all gases at the same T and P have the same number of molecules. So 22.4 L at STP always contains one mole = 6.02x10^23 molecules, no matter which gas.
1 atm = 1.01x10^5 Pa (about 101 kPa), which is also 760 mm of mercury. When a numerical gives R = 8.31 J/mol/K, you must plug pressure in pascal, not atm, so use 1.01x10^5 Pa.
They are close but not identical. STP in NCERT physics uses 273 K (0 C) and 1 atm. NTP (Normal Temperature and Pressure) commonly uses 293 K (20 C) or 298 K (25 C) at 1 atm. For this Kinetic Theory chapter stick to STP = 273 K, 1 atm.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Standard Temperature = 0 C = 273.15 K (taken as 273 K in most problems). Standard Pressure = 1 atm = 1.01x10^5 Pa = 760 mm Hg.
22.4 litres per mole for any ideal gas. This equals 22.4x10^-3 m^3, and it contains 6.02x10^23 molecules (Avogadro number).
Number density n = N_A / molar volume = 6.02x10^23 / (22.4x10^-3) = 2.7x10^25 molecules per cubic metre. This value (Loschmidt number) is worth memorising.
Divide volume by molar volume: 44.8 / 22.4 = 2 moles. NCERT uses exactly this in the helium heat problem (Example 12.8).
No. 22.4 L per mole is only at STP (273 K, 1 atm). At any other T or P you must recompute V = nRT/P.
