Chemistry · Some Basic Concepts Of Chemistry · NEET
Both are correct, but for different STP definitions. The NEW STP (0 degC = 273.15 K and 1 bar pressure) gives molar volume = 22.7 L per mole. The OLD STP (0 degC and 1 atm pressure) gives 22.4 L per mole. NCERT and recent NEET use the new value 22.7 L. So in your exam today, use 22.7 L unless the paper clearly states 22.4 L or mentions 1 atm.
Only the pressure part of STP changed, not the gas. Old STP used 1 atm (101325 Pa). New STP uses 1 bar (100000 Pa), which is a slightly lower pressure. Lower pressure means the gas spreads out a little more, so the same one mole now takes a bit more space: 22.7 L instead of 22.4 L. The temperature stayed the same at 0 degC (273.15 K).
No. This is the key idea. At the same temperature and pressure, one mole of ANY ideal gas fills the same volume. So 1 mole of H2, 1 mole of CO2, and 1 mole of He all take 22.7 L at STP. This comes from Avogadro's law: equal volumes of gases at the same T and P have equal numbers of molecules. The heavy gas and the light gas fill the same room.
STP means Standard Temperature and Pressure. The current NCERT STP is 273.15 K (0 degC) and 1 bar, giving molar volume 22.7 L. NTP (Normal Temperature and Pressure) usually means 293.15 K (20 degC) and 1 atm, which gives about 24 L per mole. NEET questions almost always say STP, so stick with 22.7 L there.
Use the simple rule: moles = volume of gas (in L) / molar volume. So moles = V / 22.7 (new STP) or V / 22.4 (old STP). Example: 11.35 L of oxygen at new STP = 11.35 / 22.7 = 0.5 mol. This only works for GASES, and only at STP. For liquids or solids you must use moles = mass / molar mass instead.
No, never. Molar volume 22.7 L is ONLY for gases at STP. Water, salt, and other liquids or solids are packed tightly, so one mole of them does NOT take 22.7 L. This is a very common NEET trap: they give you volume of water in mL and hope you divide by 22.7. For liquids and solids, always convert using density and molar mass, not molar volume.
The highest number of helium atoms is present in
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Use 22.7 L, since NCERT now defines STP as 0 degC and 1 bar. The NEET 2024 helium question used 22.71 L, so 22.7 L is the safe default. Only switch to 22.4 L if the question clearly mentions 1 atm or old STP.
At new STP it is 22.711 L per mole (often rounded to 22.7 L). At old STP (1 atm) it is 22.414 L per mole (rounded to 22.4 L). NEET usually accepts the rounded values 22.7 L and 22.4 L.
No. The values 22.4 L and 22.7 L are only true at STP (0 degC). At other conditions you must use the ideal gas equation PV = nRT to find the volume of one mole.
From PV = nRT with n = 1 mol, T = 273.15 K, and P = 1 bar (100000 Pa), using R = 8.314 J/mol/K. This gives V = RT/P = about 22.7 L. Using P = 1 atm instead gives 22.4 L.
Because of Avogadro's law: at the same temperature and pressure, equal volumes of any gas hold equal numbers of molecules. So one mole (same number of molecules) of any gas fills the same volume, no matter how heavy the gas is.
