Solving the Mystery of STP Conditions?

Do you know what is STP & what are the parameters of STP Conditions?

Questions in physical chemistry are often filled with the term STP. But most of the students, teachers, and authors have been making a very basic mistake over the years is using the correct parameters of STP.

Standard temperature and pressure are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure and Applied Chemistry (IUPAC) and the National Institute of Standards and Technology (NIST), although these are not universally accepted standards. Other organizations have established a variety of alternative definitions for their standard reference conditions.

In chemistry, IUPAC changed the definition of standard temperature and pressure (STP) in 1982

• Until 1982, STP was defined as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of exactly 1 atm (101.325 kPa).
• Since 1982, STP has been defined as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of exactly 105 Pa (100 kPa, 1 bar).

Now, most of the books are still using 1 atm pressure in STP calculations but actually, it’s 1 bar. I believe they might have not made the inevitable change because of too many calculation changes.

Now, if 1 Bar is equal to 0.98692326671 atmospheric pressure. How much difference does it make to calculation?

Citing an example from a famous book 1 mol of an ideal gas at STP using 1 atm as pressure(PV=nRT) is around 22.4 liters. But if we use the correct parameter(1 bar pressure) The volume of 1 mol of an ideal gas is around 22.7 liters. This question has been asked in a few exams and the majority of the students still write 22.4 liters as the answer. Just imagine if there are two options 22.4 and 22.7 and maximum students go with 22.4(Resulting in negative marks).

Also remember, STP should also not be confused with the standard state commonly used in thermodynamic evaluations of the Gibbs energy of a reaction.

Happy Learning!