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General Chemistry I · Gases

Standard Temperature and Pressure (STP)

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Study tools
  7. Sources & references

In 30 seconds

STP ("standard temperature and pressure") is an agreed-upon reference condition for reporting gas properties. The key trap is that "STP" has more than one convention: the classic teaching definition uses 0 °C and 1 atm, while the modern IUPAC definition uses 0 °C and 100 kPa (1 bar). Because the molar volume depends on the exact pressure chosen, the familiar "22.4 L/mol" applies only to 0 °C and 1 atm — it is not a universal constant.

Why this matters

STP lets chemists quote "a liter of gas" unambiguously and compare reaction stoichiometry, densities, and molar volumes across experiments and textbooks. Knowing which STP convention a problem uses (1 atm vs 1 bar) prevents off-by-3% errors on molar-volume and density problems.

The college version

Key Ideas

  • Classic STP: 0 °C (273.15 K) and 1 atm (101.325 kPa) → molar volume ≈ 22.4 L/mol.
  • IUPAC STP (since 1982): 0 °C (273.15 K) and 100 kPa (1 bar) → molar volume ≈ 22.7 L/mol.
  • SATP: 25 °C (298.15 K) and 100 kPa → molar volume ≈ 24.8 L/mol.
  • 22.4 L/mol is NOT universal — it is specific to 0 °C and 1 atm, for an ideal gas.
  • Molar volume = V/n, computed from the ideal gas law for whatever T and P are given.

Equations and Variables

  • Molar volume: Vₘ = V/n = RT/P.
  • Ideal gas law: PV = nRT, with R = 0.08206 L·atm/(mol·K).
  • At 0 °C (273.15 K), 1 atm: Vₘ = (0.08206 L·atm/mol·K)(273.15 K)/(1 atm) = 22.4 L/mol.
  • At 0 °C, 1 bar = 0.98692 atm: Vₘ = (0.08206)(273.15)/(0.98692) = 22.7 L/mol.
  • Density at a given T, P: d = PM/RT.

How It Works

  1. Standard conditions are arbitrary reference points chosen so scientists can compare gas volumes and densities "on the same footing."
  2. Whatever T and P you adopt, the molar volume follows from PV = nRT with n = 1 mol.
  3. At 0 °C and 1 atm, 1 mol of any ideal gas occupies 22.414 L.
  4. If the "standard" pressure is changed to 1 bar, the same mole occupies a slightly larger 22.71 L.
  5. Therefore the molar volume is a calculation, not a constant — it changes whenever the definition of STP changes.

Worked Example

How many moles of O₂ are in 22.4 L at STP (defined as 0 °C and 1 atm)? n = PV/RT = (1 atm)(22.4 L) / [(0.08206 L·atm/mol·K)(273.15 K)] = 1.00 mol.

What mass of O₂ (M = 32.0 g/mol) is that? mass = n × M = 1.00 mol × 32.0 g/mol = 32.0 g.

What is the density of O₂ at STP (0 °C, 1 atm)? d = PM/RT = (1 atm)(32.0 g/mol) / [(0.08206 L·atm/mol·K)(273.15 K)] = 1.43 g/L.

Common Confusions

  • "22.4 L/mol is a universal constant" — it is not; it holds only at 0 °C and 1 atm. At other conditions (or IUPAC STP) the molar volume is different.
  • "STP is always 1 atm" — IUPAC defines STP as 100 kPa (1 bar), giving 22.7 L/mol; many textbooks still use 1 atm. Always check which convention is specified.
  • "22.4 L/mol works at room temperature" — no; at 25 °C the molar volume is larger (about 24.5 L at 1 atm, or 24.8 L at 1 bar).
  • "A mole of liquid water also occupies 22.4 L" — no; 22.4 L/mol applies only to gases (1 mol of liquid water is only ~18 mL).
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

"STP" is like agreeing on a standard weather report before comparing cars' gas mileage — you need the same road conditions to make a fair comparison. The classic version is "freezing cold (0 °C) at normal sea-level pressure (1 atm)," where a mole of gas takes up about 22.4 L. But some rule books changed "normal pressure" slightly, and then a mole takes up 22.7 L instead. The analogy's limit: there's no "true" STP — it's a human convention, so the number you get depends on which convention the problem tells you to use.

Key takeaways

  • Classic STP: 0 °C (273.15 K), 1 atm → 22.4 L/mol.
  • IUPAC STP: 0 °C, 100 kPa (1 bar) → 22.7 L/mol.
  • SATP: 25 °C, 100 kPa → 24.8 L/mol.
  • 22.4 L/mol is valid only at 0 °C and 1 atm (ideal gas).
  • Molar volume Vₘ = RT/P is always computed, never memorized as a constant.
  • 1 mol of any gas at the same T and P occupies the same volume (Avogadro's law).
  • STP is a reference condition; two common definitions exist.
  • Classic STP (0 °C, 1 atm): molar volume 22.4 L/mol.
  • IUPAC STP (0 °C, 100 kPa): molar volume 22.7 L/mol.
  • Molar volume = RT/P, always computed for the given T and P.
  • 22.4 L/mol is not universal — never apply it blindly.

Keep learning

Ready to build on this? Continue to the next lesson.

Practice General Chemistry I

This lesson has no separate scored set. Practice draws from the subject’s question bank.

Study tools & related lessonsYou’ll learn to · Related

You’ll learn to

  • Define STP and list the different conventions in use.
  • Explain why 22.4 L/mol is not a universal constant.
  • Calculate molar volume at a specified temperature and pressure.
  • Convert between volume, moles, and mass at STP.

Sources & references

  1. OpenStax, "9.2 Relating Pressure, Volume, Amount, and Temperature," Chemistry 2e.
  2. Petrucci et al., "6.4 Applications of the Ideal Gas Equation," Chemistry LibreTexts.
  3. NIST CODATA, "molar gas constant."

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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