Question

Calculate the fugacity and fugacity coefficient of saturated liquid water at 25 degrees.

Answer 1

Fugacity is simply a measure of molar Gibbs energy of a real gas . Modifying the simple equation for the chemical potential of an ideal gas by introducing the concept of a fugacity (f). The fugacity is an “ effective pressure” which forces the equation below to be true for real gases: µ = µ^θ T+ RT ln(f/p^θ) where pθ = 1 atm

If fugacity is an “effective pressure” i.e, the pressure that gives the right value for the chemical potential of a real gas. So, the only way we can get a value for it and hence for µ is from the gas pressure. Thus we must find the relation between the effective pressure f and the measured pressure p.

let f = φ p

φ is defined as the fugacity coefficient. φ is the “fudge factor” that modifies the actual measured pressure to give the true chemical potential of the real gas.

For a pure fluid in vapor–liquid equilibrium, the vapor phase fugacity is equal to the liquid phase fugacity. At pressures above the saturation pressure, the liquid phase fugacity is:

where v_liq is the molar volume of the liquid.

The fugacity correction factor for the vapor, φ_sat, should be evaluated at the saturation pressure and is unity when P_sat is low. The exponential term represents the Poynting correction factor and is usually near 1 unless pressures are very high.

lnϕ= BP/ RT = -233.3 *(1)/ 8.314 *298 =-233.3/2477.572 = 0.094165

= 0.094165/1atm = 0.094165