Standard Temperature and Pressure

Before you can compare gas behavior across different experiments, you need a common reference point. Standard Temperature and Pressure (STP) provides that baseline, and it comes with one of the most useful numbers in all of MCAT chemistry: the molar volume of an ideal gas.

The STP Definition

Molar Volume at STP

At STP, one mole of ANY ideal gas occupies exactly 22.4 liters. This is true regardless of the gas’s identity - one mole of helium, one mole of oxygen, and one mole of sulfur hexafluoride all occupy 22.4 L at STP.

This follows directly from the ideal gas law:

V = nRT/P = (1 mol)(0.0821 L·atm/mol·K)(273 K) / (1 atm) = 22.4 L

Why Molar Volume Is Identity-Independent

The molar volume being the same for all ideal gases is a direct consequence of Avogadro’s hypothesis: equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. At STP, 22.4 L always contains $6.022 \times 10^{23}$ molecules, whether they are tiny helium atoms or large SF₆ molecules.

This seems counterintuitive - how can a mole of heavy gas take up the same space as a mole of light gas? The answer is that in the gas phase, molecules are so far apart that their actual size is negligible compared to the empty space between them. The volume of a gas is determined by how much space the molecules MOVE through, not how much space they physically occupy.

Using Molar Volume as a Shortcut

Example: How many liters does 0.5 mol of N2 occupy at STP?

0.5 mol x 22.4 L/mol = 11.2 L

Example: What is the mass of 44.8 L of CO2 at STP?

44.8 L / 22.4 L/mol = 2.0 mol CO2

2.0 mol x 44 g/mol = 88 g

Standard Conditions vs. Standard State

Do not confuse STP with “standard state” conditions used in thermodynamics:

• STP (gas laws): 0 C (273 K), 1 atm
• Standard state (thermodynamics): 25 C (298 K), 1 atm, 1 M concentrations

The MCAT can use either depending on the context. Gas law problems usually specify STP. Thermodynamics problems use standard state (25 C).