The solubility product constant (Ksp) is a key concept in understanding how ionic compounds dissolve in water. It helps us predict and compare the solubility of different salts, which is crucial in many chemical processes and environmental systems.
Ksp calculations allow us to determine molar solubility and compare relative solubilities of compounds. pH can greatly affect salt solubility, with some salts becoming more soluble in acidic conditions and others in basic conditions.
Solubility Product Constant (Ksp) and Molar Solubility
Definition of solubility product constant
- Quantitative measure of the equilibrium between a sparingly soluble ionic compound and its dissolved ions in an aqueous solution
- Calculated as the product of the concentrations of the dissolved ions raised to the power of their stoichiometric coefficients in the balanced chemical equation
- Represents the maximum concentration of ions that can be present in a saturated solution at a given temperature
- Denoted by the symbol Ksp and expressed without units (e.g., Ksp for silver chloride is $1.8 \times 10^{-10}$)
- Smaller Ksp values indicate lower solubility while larger Ksp values indicate higher solubility of the compound (e.g., AgCl has a lower solubility than CuCl)
Calculations with Ksp
- Calculate Ksp from molar solubility by writing the balanced dissolution reaction, expressing ion concentrations in terms of molar solubility, and substituting into the Ksp expression
- For example, for $CaF_2$: $CaF_2 \rightleftharpoons Ca^{2+} + 2F^-$, if molar solubility is s, then $Ksp = [Ca^{2+}][F^-]^2 = s(2s)^2 = 4s^3$
- Determine molar solubility from Ksp by writing the balanced dissolution reaction, expressing Ksp in terms of molar solubility, and solving for molar solubility
- For example, for $Ag_2CrO_4$: $Ag_2CrO_4 \rightleftharpoons 2Ag^+ + CrO_4^{2-}$, if Ksp is known, then $Ksp = [Ag^+]^2[CrO_4^{2-}] = (2s)^2(s) = 4s^3$, solve for s
Relative solubility from Ksp
- Compare solubilities of compounds based on their Ksp values at a given temperature
- Compounds with similar Ksp values have similar solubilities (e.g., $AgCl$ and $PbCl_2$)
- Solubility decreases as Ksp value decreases (e.g., $BaSO_4$ is less soluble than $CaSO_4$)
- Solubility increases as Ksp value increases (e.g., $CuCl$ is more soluble than $AgCl$)
- Rank compounds in order of increasing or decreasing solubility based on their Ksp values
pH effects on salt solubility
- Solubility of some salts affected by solution pH due to presence of ions that are conjugate bases of weak acids or conjugate acids of weak bases
- Salts with anions that are conjugate bases of weak acids (e.g., $CO_3^{2-}, PO_4^{3-}$) exhibit decreased solubility as pH increases (more basic) and increased solubility as pH decreases (more acidic)
- For example, solubility of $CaCO_3$ decreases in basic solutions
- Salts with cations that are conjugate acids of weak bases (e.g., $NH_4^+$) show increased solubility as pH increases (more basic) and decreased solubility as pH decreases (more acidic)
- For example, solubility of $AgCl$ increases in the presence of $NH_3$
- Le Chatelier's principle explains pH effects on solubility: adding or removing ions (by changing pH) shifts equilibrium to counteract the change and restore balance