How this calculator works
For aA + bB in equilibrium with cC + dD, multiply product concentrations raised to their balanced coefficients and divide by the analogous reactant product. Concentrations are normalized to 1 mol/L; this is the ideal dilute-solution approximation to an activity equilibrium constant. A zero coefficient omits a species, including pure solids and liquids. Enter equilibrium concentrations, not starting amounts. Solving one species assumes all other equilibrium values are already known.
Formula
Kc = ([C]/c0)^c ([D]/c0)^d / (([A]/c0)^a ([B]/c0)^b), c0 = 1 mol/L.
Worked example
For A + B in equilibrium with C, equilibrium values 1 M, 2 M and 6 M give Kc = 3. For A + B in equilibrium with 2C, Kc = 4 and [A] = [B] = 1 M imply [C] = 2 M.
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Frequently asked questions
Should I include a pure solid?
Omit it by setting its coefficient to zero in this expression; its activity is approximately one.
Can K be negative?
No. An equilibrium constant formed from positive activities is positive.
Can I use initial concentrations?
Only after an equilibrium calculation has established that they are also equilibrium values.