Standard state definitions for thermodynamics are typically 1 bar for gases and 1 M for solutes. True or false?

• A. True
• B. False
• C. 1 atm for gases and 1 M for solutes
• D. 1 bar for all species

Answer: (A)

Standard states set the reference point for chemical potentials. For gases, the reference pressure is 1 bar, so the standard-state chemical potential μ° for a gas is defined at 1 bar. For solutes in solution, the reference is a concentration of 1 M, so μ° for a dissolved species is defined at 1 M. The dependence of the chemical potential on state is given by μ = μ° + RT ln a, where a is the activity. For gases, activity is roughly proportional to pressure (P/1 bar), and for dilute solutions, it’s proportional to concentration ([C]/1 M). This is why standard states are commonly taken as 1 bar for gases and 1 M for solutes. Note that older data often used 1 atm for gases, but modern convention favors 1 bar for consistency.