Standard temperature and pressure (STP)

Standard temperature and pressure (STP) is the agreed reference temperature and pressure at which the molar volume of a gas is 22.4 dm³ mol⁻¹. Malay: suhu dan tekanan piawai (STP) · Chinese: 标准状况（STP）.

Because the volume of a gas changes with temperature and pressure, chemists need agreed reference conditions so that gas volumes can be compared and calculated consistently. A colder or more compressed gas takes up less space, so a volume only means something definite once the conditions are fixed. STP is one such standard reference, and the value of the molar volume you use in a calculation depends on which reference the question specifies. Whenever a question says the gas is at STP, you take the molar volume as 22.4 dm³ mol⁻¹ rather than the room-conditions value.

Example. One mole of any gas at STP occupies 22.4 dm³ mol⁻¹, so two moles of oxygen at STP occupy 2 × 22.4 dm³ mol⁻¹. If a question gives you a gas volume at STP, you find the number of moles by dividing that volume by 22.4 dm³ mol⁻¹.

Confusion to avoid. The most common mistake is mixing up STP with room conditions. At room conditions the molar volume is 24 dm³ mol⁻¹, while at STP it is 22.4 dm³ mol⁻¹; using the wrong value gives the wrong answer even when the method is correct. Always read the question to see which conditions apply, and quote the matching molar volume.

In this chapter STP is one of the two standard sets of conditions that fix the molar volume, the other being room conditions. It works hand in hand with the mole and Avogadro’s law whenever a calculation involves the volume of a gas, and reading the stated conditions correctly is a small habit that protects easy marks across SPM Chemistry.