Keq is generally considered to be dimensionless under standard conditions. Which statement best captures this?

• A. Keq has units that depend on the number of moles in the balanced equation.
• B. Keq is dimensionless under standard state conditions.
• C. Keq always has units of concentration.
• D. Keq cannot be compared between reactions.

Answer: (B)

The main idea is that equilibrium constants are defined from activities, and when activities are measured relative to standard states they become dimensionless. For solutions, each activity is defined as a_i = [i]/c°, with c° = 1 M; for gases, a_i = P_i/P°, with P° = 1 atm. The equilibrium constant expression uses these activities in ratios, so every factor is a dimensionless ratio to its standard state. Multiply and divide as required by the balanced equation, and the units cancel, giving Keq as a pure number under standard conditions.

That’s why the statement stating Keq is dimensionless under standard state conditions is the best choice. It reflects the convention that standard-state activities set the scale, making the entire ratio unitless. In contrast, thinking Keq always has units of concentration isn’t consistent with the standard-state definition, and while you could construct expressions with units if you ignore standard states, the standard convention yields a dimensionless quantity. Also, because Keq is dimensionless, you can compare equilibrium positions across reactions more meaningfully.