Standard state pressure

Every standard enthalpy of formation, standard entropy and equilibrium constant is quoted relative to a reference pressure, and since 1982 IUPAC's is exactly 1 bar = 100 000 Pa. The superscript degree symbol in ΔG° means "at p° = 1 bar", and the partial pressures inside an equilibrium constant are divided by it, which is why K is dimensionless.

Older tables use 1 atm = 101.325 kPa, a 1.3 % difference. For most reactions it shifts ΔG° by only tens of joules per mole, but it changes tabulated standard entropies of gases by R ln(1.01325) ≈ 0.109 J/(mol·K), enough to matter in precise work. Standard state fixes pressure only; the temperature is whatever the table says, conventionally 298.15 K.