How this calculation works
Equal molar amounts of monovalent and divalent ions do not contribute equally to ionic strength. Squared charge gives a divalent ion four times the contribution per mole, which matters when approximating dilute-solution activity corrections. This page builds ionic strength from an entered species list and applies the stated Davies approximation at twenty-five degrees Celsius. It also reports signed charge imbalance so an incomplete list is visible.
I = ½Σ(c z²); log10 gamma = −0.509 z²[√I/(1+√I) − 0.3I]A worked example
Calcium at 0.01 molar with charge plus two contributes 0.02 molar to ionic strength. Chloride at 0.02 molar with charge minus one contributes another 0.01, giving a total of 0.03. Their signed charge contributions cancel. The divalent activity coefficient is lower than the monovalent coefficient under the same entered conditions because the Davies exponent depends on squared charge.
Good to know
Specific ion interactions, complex formation and ion pairing are not modeled. Species with equal charge receive the same coefficient even if their real chemical behavior differs substantially. Molality and molarity are not interconverted, and temperature is fixed. For concentrated solutions or accurate equilibrium work, select a validated activity model and composition data appropriate to the actual solution.
Reference: Carnegie Mellon Kitchin group — Davies speciation model (opens in a new tab) ↗
What this calculation answers
Equal molar amounts of monovalent and divalent ions do not contribute equally to ionic strength. Squared charge gives a divalent ion four times the contribution per mole, which matters when approximating dilute-solution activity corrections.
This page builds ionic strength from an entered species list and applies the stated Davies approximation at twenty-five degrees Celsius. It also reports signed charge imbalance so an incomplete list is visible.
The Molarity calculator converts an amount and solution volume into formal concentration. Ionic strength additionally requires the separate ion charges and their squared contributions.
Use the right input conventions
Provide dissolved ionic species separately, with concentration in moles per litre and signed integer charge. A salt formula is not automatically dissociated into its constituent ions by the calculator.
The approximation uses concentration-based ionic strength and the coefficient 0.509 with a 0.3I correction. It rejects totals above 0.5 molar; that ceiling describes the accepted approximation range rather than a guarantee of accurate chemistry near the upper bound.
Interpret the result
The table reports each species activity coefficient and concentration multiplied by that coefficient. The latter retains concentration units and should not be confused with a dimensionless thermodynamic activity without specifying its standard state.
Net signed concentration charge is a separate diagnostic. A nonzero balance is displayed rather than silently adding counterions, because invented ions would change both the composition and the calculated ionic strength.
Work through the example
Calcium at 0.01 molar with charge plus two contributes 0.02 molar to ionic strength. Chloride at 0.02 molar with charge minus one contributes another 0.01, giving a total of 0.03.
Their signed charge contributions cancel. The divalent activity coefficient is lower than the monovalent coefficient under the same entered conditions because the Davies exponent depends on squared charge.
Where the model stops
Specific ion interactions, complex formation and ion pairing are not modeled. Species with equal charge receive the same coefficient even if their real chemical behavior differs substantially.
Molality and molarity are not interconverted, and temperature is fixed. For concentrated solutions or accurate equilibrium work, select a validated activity model and composition data appropriate to the actual solution.
A few questions, answered
Why display charge imbalance instead of correcting it?
A missing counterion changes the solution composition. The calculator cannot choose it from an incomplete list.
What changes in the comparison?
Replace the divalent salt with a balanced monovalent pair and compare the squared-charge contribution.