Acentric factor (ω) – typical magnitude and definition insight The acentric factor ω is defined by ω = −log10(Pr_sat) at Tr = 0.7. What is the usual magnitude range of ω for real substances?

Introduction / Context:
The acentric factor (ω) is a shape- and polarity-related parameter introduced by Pitzer to improve corresponding-states correlations for vapor pressures and compressibility. It is widely used with cubic equations of state (e.g., Soave–Redlich–Kwong, Peng–Robinson).

Given Data / Assumptions:

• Definition: ω = −log10(Pr_sat) at Tr = 0.7.
• Pr_sat = Psat / Pc and Tr = T / Tc.
• Simple spherical molecules (e.g., argon, methane) have small ω; complex/polar molecules have larger ω.

Concept / Approach:
By construction, ω measures the deviation of a substance from the simple spherical model. For many common fluids, ω falls between roughly 0 and 1. Light gases like methane have ω near 0.01, while heavier or more polar substances like water are around 0.34, and aromatics may approach ~0.6–0.9. Values greater than 1 are unusual and reserved for highly non-ideal species under specific definitions; thus the safe, general statement is ω < 1 for typical substances.

Step-by-Step Solution:

Verification / Alternative check:
Databanks list ω(CH4) ≈ 0.011, ω(N2) ≈ 0.037, ω(H2O) ≈ 0.344, ω(benzene) ≈ 0.212, all less than 1.

Why Other Options Are Wrong:

• > 2 or > 1: Not representative of typical fluids.
• < 3: Trivially true but uninformative; the conventional characterization is ω well below 1.

Common Pitfalls:
Assuming ω relates directly to critical compressibility factor; although correlated, ω specifically parameterizes vapor-pressure behavior at a reduced temperature.

Final Answer:
< 1