Relate activity to normalized concentration
An activity coefficient connects thermodynamic activity with a stated normalized concentration. In direct mode, enter dimensionless activity and c/c°, the concentration divided by its standard-state concentration. The calculator returns γ = a/(c/c°). All three quantities in this relationship are dimensionless, even though the underlying concentration used to form c/c° has a physical unit.
A coefficient of one corresponds to ideal behaviour under the chosen convention. Values above or below one show deviation from that reference, but they are not percentages and do not by themselves explain the molecular cause. For activity 0.08 and normalized concentration 0.1, γ is 0.8. The result retains the convention warning because coefficients from different standard states should not be compared blindly.
Estimate a single-ion value with Davies
Davies mode applies log10γ = −0.509z²[√I/(1+√I)−0.3I] for an aqueous 25°C teaching estimate. Enter ionic strength in mol/L and a signed non-zero whole-number ion charge. The sign does not change this equation because charge is squared, so ions with charges +2 and −2 receive the same numerical estimate at the same ionic strength.
The page limits ionic strength to 0–0.5 mol/L and charges to a bounded classroom range. At zero ionic strength, the estimate tends to γ = 1. As ionic strength and charge magnitude rise, electrostatic non-ideality becomes more important. The displayed square root, bracket term, logarithm, and final power of ten let you check each part separately.
Choose the mode that matches the information
Use direct mode when activity and the matching normalized concentration are already known or supplied. Use Davies mode only when a problem explicitly asks for that approximation and provides ionic strength and ion charge. Davies does not calculate ionic strength from a composition list, and direct mode does not infer the standard state hidden behind an activity value.
Single-ion activity coefficients cannot generally be measured independently without an extra convention; many experiments determine mean ionic coefficients for an electrolyte. A classroom prompt may nevertheless request a single-ion model value. Label it as a Davies estimate rather than presenting it as a directly measured universal property, especially when comparing it with tables or software using another model.
Check the coefficient and its limits
In direct mode, multiplying γ by c/c° should reproduce activity. In Davies mode, substituting I = 0 should return one for every permitted charge. A larger absolute charge changes the logarithm with z², so a divalent estimate should depart more strongly from one than a monovalent estimate at the same ionic strength. These checks catch charge and logarithm-entry mistakes.
The Davies equation is an extension of dilute-solution electrostatic ideas, not a complete description of specific ion interactions. Concentrated electrolytes, mixed solvents, unusual temperatures, ion pairing, and high-accuracy equilibrium work can require models such as extended Debye–Hückel, SIT, Pitzer, or experimentally fitted data. Choose a model consistent with the source and intended precision.
Thermodynamic scope and privacy
This activity coefficient calculator does not predict solubility, equilibrium composition, pH, ionic strength from ingredients, or safety. Temperature is fixed at 25°C only in Davies mode. Direct results are as reliable as the activity, normalized concentration, and standard-state convention entered. Always carry that convention with a copied coefficient so the dimensionless number remains interpretable.
The values are calculated in the current browser tab. NexaCurrent does not upload the quantities, formulas, chemical names, element choices, or other information entered here, and no account is needed. Copying is a deliberate action after the answer and working are visible. Refreshing or leaving the page clears the current calculation, so keep a copied result if it is needed for later study.