Standard Temperature and Pressure
Standard temperature and pressure, or STP, are the reference conditions used in chemistry to compare gas properties. The reason for having standard conditions is that they conveniently allow consistent measurements of gas properties like mass and volume. Therefore, it is necessary to have them for scientists to use in all locations around the world to make significant comparisons between different data sets. STP should not be confused with the standard state used in Gibbs and Helmholtz free energy.
The International Union of Pure and Applied Chemistry (IUPAC) defines standard temperature and pressure as follows:
- A temperature of 0 0C or 273.15 K
- A pressure of 105 Pa or 1 bar (formerly 101.325 kPa or 1 atm, but IUPAC has changed it since 1982)
Gases behave differently at different temperatures and pressures. Some properties can change with both. Therefore, having standard conditions as reference points to compare these properties is helpful. Using the ideal gas law, PV=nRT, one can find that 1 mole of an ideal gas occupies a volume of 22.4 L at STP. This value holds irrespective of the identity of the gas.
STP finds application in numerous thermodynamic calculations and tabulations. Various characteristics of substances, including density, viscosity, and boiling point, demonstrate variability in response to alterations in temperature or pressure. Utilizing a standardized set of conditions for documenting these values enables convenient comparisons and simplifies calculations.
STP values are frequently used in gas experiments and fluid flow rate calculations. They are essential in gas stoichiometry to study the relationship between the quantities of reactants and products.
Other Standard Values
In addition to STP, other values of temperature and pressure can be used to compare gas properties. Other frequently used conditions are:
- Normal Temperature and Pressure (NTP) – Defined as a temperature of 293.15 Kelvin and pressure of 105 Pa
- Standard Ambient Temperature and Pressure (SATP) – Defined at a temperature of 298.15 K and pressure of 105 Pa
Problem 1. According to Avogadro’s law, 1 mole of any gas occupies 22.4 L at STP. Calculate the number of moles in 90 L of O2.
22.4 L of O2 is made up of 1 mole. Therefore, 90 L of O2 is made up of
1 M/22.4 L x 90 L = 4.02 M
Ans. The air density at STP is 1.2754 kg/m3.