11.3.2 Equation of State for an Ideal Gas

It is impossible to track and analyse each and every atom or molecule in a system. Fortunately, a set of state variables, namely the pressure p, volume V and temperature T, can be used to describe the state of the system. An equation used to model the relationship among the state variables (of a given amount of substance) is called an equation of state.

The equation of state for an ideal gas is


where n is the amount of substance in moles, and R=8.31\text{ J }{{\text{K}}^{{-1}}}\text{ mo}{{\text{l}}^{{-1}}}  is the molar gas constant.

Since each mole contains {{N}_{A}}=6.02\times {{10}^{{23}}}  particles (called the Avogadro’s number), the equation of state can also be written as


where  N is the number of gas particles, and \displaystyle k=\frac{R}{{{{N}_{A}}}}=1.38\times {{10}^{{-23}}}\text{ J }{{\text{K}}^{{-1}}}  is the Boltzmann’s constant.

  • When plugging numbers into this equation, one must ensure that p is the absolute pressure (not pressure relative to atmospheric pressure, aka gauge pressure), and T must be the thermodynamic temperature in Kelvin (not Celsius).
  • Historically, the three empirical gas laws, namely

Boyle’s Law: {{p}_{1}}{{V}_{1}}={{p}_{2}}{{V}_{2}}  at constant T

Charles’ Law: \displaystyle \frac{{{{V}_{1}}}}{{{{T}_{1}}}}=\frac{{{{V}_{2}}}}{{{{T}_{2}}}}  at constant p

Gay-Lussac’s Law: \displaystyle \frac{{{{p}_{1}}}}{{{{T}_{1}}}}=\frac{{{{p}_{2}}}}{{{{T}_{2}}}}  at constant V

were first combined into what’s called the

Combined Gas Law: \displaystyle \frac{{{{p}_{1}}{{V}_{1}}}}{{{{T}_{1}}}}=\frac{{{{p}_{2}}{{V}_{2}}}}{{{{T}_{2}}}}

which is then combined with

Avogadro’s Law: \displaystyle \frac{{{{V}_{1}}}}{{{{n}_{1}}}}=\frac{{{{V}_{2}}}}{{{{n}_{2}}}}  at constant p and T

to finally arrive at



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