I am trying to understand what fugacity really is. I asked a related question on chemistry stack exchange but am not satisfied with the answer. In many books, such as Physical Chemistry by Engel and Reid, Physical chemistry bh Castellan, etc. fugacity is called as effective pressure. My questions are

a)What does the word 'effective' is trying to say? and b) If we already have various equation of states that have already accounted for intermolecular forces, then why is there a need for fugacity?

I'm sorry if this is a stupid question but english is not my first language and maybe I am not interpreting fugacity correctly. So any help is appreciated!


1 Answer 1


Fugacity = the pressure of an ideal gas with the same chemical potential as the real gas

  • $\begingroup$ Then why don't we use that pressure of real gas where we get the chemical potential we want? $\endgroup$
    – Natasha J
    Commented Jul 24, 2023 at 15:11
  • $\begingroup$ I don't fully understand the comment, but using fugacity allows us to utilize ideal gas relationships like law of partial pressures (i.e. partial fugacities), equilibrium constant $Kp$, etc. that are not valid for non-ideal gases. $\endgroup$
    – RC_23
    Commented Jul 24, 2023 at 15:48
  • $\begingroup$ I'm sorry for being unclear, but your answer cleared my doubt. Thank you very much. $\endgroup$
    – Natasha J
    Commented Jul 24, 2023 at 16:49
  • $\begingroup$ @ RC_23, One more thing, if fugacity considers all the non-idealness of real gases, then wouldn't thermodynamical models using pressure(instead of fugacity) lead to erroneous results? $\endgroup$
    – Natasha J
    Commented Jul 24, 2023 at 16:58
  • $\begingroup$ They do lead to erroneous results if the deviations from ideal gas behavior are significant enough. Many gases studied in everyday life (e.g. air at pressures below ~3000 psi/20 MPa, power and engine cycle gases) are well-modeled as ideal gases with negligible errors. So fugacity would be equal to pressure in these cases for all intents and purposes. $\endgroup$
    – RC_23
    Commented Aug 9, 2023 at 15:46

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