# A rule of thumb when assuming ideal gas

When to assume ideal gas law without incurring too intolerable error. I have the idea that it must be at high temperature and low pressure. Is there any specific values, and are there any other things to look for?

• Is your idea of "too intolerable" an error of 10% or 0.1%? Air at room temperature is near enough an ideal gas for many purposes, so "high temperature and low pressure" are rather subjective terms. Jul 26 '17 at 22:43
• One of my professors said the ideal gas law is generally good enough for molar volumes over 20 L/mol. Sep 30 '17 at 14:04

One way to describe how gasses deviate from ideal gasses is by the compressibility factor. Compressibility is defined as: $$Z=\frac{PV}{nRT}$$ For an ideal gas, Z = 1, which leads to the familiar ideal gas law: $$PV=nRT$$ You can look of various compressibility factors for air at different temperatures and pressures. When the compressibility deviates from 1.0, air is not behaving like an ideal gas. You can find a table of values here: http://www.enggcyclopedia.com/2011/09/air-compressibility-factor-table/