Consider dissolving of the slightly soluble ionic solid CaF2 to give its constitutent ions. The change in concentration of the ions due to dissolving is related to the molar solubility (s) through the coefficients in the balanced equation.
Ks (the solubility constant) is equal to the reaction quotient when the system is at equilibrium (in the presence of excess solid).
Ks can be calculated from the experimental molar solubility s(CaF2) in any aqueous solution by
| s(CaF2) | = | change in [Ca2+] | = | change in [F–] |
| 1 | 2 |
Ks = [Ca2+][F–]2
reaction quotient
reaction quotient
CaF2(s) 
Ca2+(aq) + 2F–(aq)
Ca2+(aq) + 2F–(aq)reactant-favoured so uses equilibrium arrow
Ks (the solubility constant) is equal to the reaction quotient when the system is at equilibrium (in the presence of excess solid).
Note that the reaction quotient expression does not include the solid.
This is because in a solution at equilibrium, as long as solid is present, changing the amount of solid does NOT affect the equilibrium concentrations of the ions.
This is because in a solution at equilibrium, as long as solid is present, changing the amount of solid does NOT affect the equilibrium concentrations of the ions.
Ks can be calculated from the experimental molar solubility s(CaF2) in any aqueous solution by
- relating the change in concentration of ions due to dissolving to the experimental molar solubility using the dissolving equation (as at the right above)
- determining the equilibrium concentration of the ions by adding this change to their initial concentration.
If the only source of an ion is dissolving (as when the solid is dissolved in water), the initial concentration is zero. - Substituting the expressions involving molar solubility into the reaction quotient expression.