Chemistry CFE Practice Test 2026 – Complete Exam Prep

R is the gas constant, a proportionality constant that makes the ideal gas law work across different units and meaningfully connect pressure, volume, temperature, and amount of substance. It sets the scale so PV = nRT holds for any ideal gas when you use consistent units. In SI units it’s about 8.314 J/(mol·K); in liter-atm units it’s about 0.0821 L·atm/(mol·K). It’s not the rate constant from kinetics, nor the gravitational constant, nor the molar mass of a particular gas. The constant serves as a universal bridge between the macroscopic state variables and the amount of substance.