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The compression factor

المؤلف:  Peter Atkins، Julio de Paula

المصدر:  ATKINS PHYSICAL CHEMISTRY

الجزء والصفحة:  ص14-15

2025-10-29

518

The compression factor

The compression factor, Z, of a gas is the ratio of its measured molar volume, Vm = V/n, to the molar volume of a perfect gas, Vo m, at the same pressure and temperature:

Because the molar volume of a perfect gas is equal to RT/p, an equivalent expression is Z =RT/pV^o_m, which we can write as

pV_m=RTZ                                                                          (1.17)

Because for a perfect gas Z = 1 under all conditions, deviation of Z from 1 is a measure of departure from perfect behaviour. Some experimental values of Z are plotted in Fig. 1.14. At very low pressures, all the gases shown have Z ≈ 1 and behave nearly perfectly. At high pressures, all the gases have Z > 1, signifying that they have a larger molar volume than a perfect gas. Repulsive forces are now dominant. At intermediate pressures, most gases have Z < 1, indicating that the attractive forces are reducing the molar volume relative to that of a perfect gas.