SOLUBILITY AND SOLUBILITY PRODUCT
Solubility refers to the ability of a substance, known as the solute, to dissolve in a solvent to form a solution. The solubility of a substance can be measured in terms of the amount of solute that dissolves in a given amount of solvent at a particular temperature and pressure.
Solubility products, on the other hand, are equilibrium constants that describe the extent to which a solid solute will dissolve in a solvent to form a saturated solution. The solubility product (Ksp) is defined as the product of the concentrations of the ions in a saturated solution, each raised to the power of their stoichiometric coefficient in the balanced chemical equation representing the dissolution process.
Solubility products of ionic compounds
Sparingly soluble ionic compounds are those that have a low solubility in water. When such compounds are added to water, only a small fraction of the solid will dissolve, and the rest will remain as a solid.
The solubility of these ionic compounds is determined by their solubility product constant, also known as Ksp. Ksp is a measure of the extent to which an ionic compound will dissolve in water, and is defined as the product of the concentrations of the ions in a saturated solution, each raised to the power of its stoichiometric coefficient in the balanced chemical equation.
For example, consider the sparingly soluble ionic compound silver chloride (AgCl), which can be represented by the following balanced chemical equation:
AgCl(s) ⇌ Ag+(aq) + Cl^-(aq)
The solubility product constant for AgCl is given by:
Ksp = [Ag+][Cl^-]
where [Ag+] and [Cl^-] represent the concentrations of Ag+ and Cl^- ions in a saturated solution of AgCl.
The value of Ksp for AgCl can be determined experimentally by measuring the solubility of the compound in water and using that value to calculate the concentrations of the ions in solution. The value of Ksp for AgCl is approximately 1.8 × 10^-10 at room temperature, indicating that AgCl is a very sparingly soluble compound in water.
If the concentration of either ion in solution is increased above the value predicted by Ksp, then the excess ion will combine with the other ion to form more of the sparingly soluble ionic compound. Conversely, if the concentration of either ion is decreased below the value predicted by Ksp, then some of the ionic compound will dissolve until the concentrations of the ions are in equilibrium with the solid compound.
In summary, Ksp is a measure of the solubility of sparingly soluble ionic compounds in water. The value of Ksp depends on the nature of the compound and the temperature of the solution, and can be used to predict the solubility of the compound under different conditions.
Solubility Calculations
Here are some examples of calculations involving solubility and solubility products:
- Calculating solubility from Ksp: Suppose the solubility product of silver chloride (AgCl) is 1.6 x 10^-10 at 25°C. What is the solubility of AgCl in water at this temperature?
AgCl ⇌ Ag+ + Cl- Ksp = [Ag+][Cl-] = 1.6 x 10^-10
Assuming x is the concentration of Ag+ and Cl- ions in the saturated solution, we can write:
Ksp = x^2
x = √(Ksp) = √(1.6 x 10^-10) = 1.3 x 10^-5 mol/L
Therefore, the solubility of AgCl in water at 25°C is 1.3 x 10^-5 mol/L.
- Calculating Ksp from solubility: Suppose the solubility of calcium sulfate (CaSO4) in water at 25°C is 4.9 x 10^-5 mol/L. What is the solubility product of CaSO4 at this temperature?
CaSO4 ⇌ Ca2+ + SO42- Ksp = [Ca2+][SO42-]
Assuming x is the concentration of Ca2+ and SO42- ions in the saturated solution, we can write:
[Ca2+] = x [SO42-] = x
Ksp = x^2
x = 4.9 x 10^-5 mol/L
Ksp = (4.9 x 10^-5)^2 = 2.4 x 10^-9
Therefore, the solubility product of CaSO4 at 25°C is 2.4 x 10^-9.
- Calculating the molar solubility of a salt with a common ion: Suppose we have a solution of silver chloride (AgCl) with a concentration of 0.010 M. What is the molar solubility of AgCl in this solution? The Ksp of AgCl is 1.6 x 10^-10 at 25°C.
AgCl ⇌ Ag+ + Cl- Ksp = [Ag+][Cl-] = 1.6 x 10^-10
Assuming x is the molar solubility of AgCl in the presence of 0.010 M Cl-, we can write:
[Ag+] = x [Cl-] = 0.010 + x
Ksp = x(0.010 + x)
Solving for x:
x^2 + 0.010x – 1.6 x 10^-10 = 0
Using the quadratic formula, we get:
x = 1.3 x 10^-6 M
Therefore, the molar solubility of AgCl in the presence of 0.010 M Cl- is 1.3
Solubility Factors
The solubility of a substance depends on several factors, including:
- Nature of the solute and solvent: The chemical nature of the solute and solvent affects solubility. Polar solutes dissolve in polar solvents, while nonpolar solutes dissolve in nonpolar solvents. For example, sugar, a polar molecule, dissolves in water, a polar solvent, but not in oil, a nonpolar solvent.
- Temperature: Solubility generally increases with an increase in temperature. This is because an increase in temperature increases the kinetic energy of the molecules, making them more likely to interact and form a solution.
- Pressure: The solubility of gases in liquids increases with an increase in pressure. This is because the increased pressure forces more gas molecules into the liquid, increasing the chance of interactions and solubility.
- Surface area: The solubility of a solid solute increases with an increase in its surface area. This is because increasing the surface area exposes more solute molecules to the solvent, increasing the likelihood of interaction.
- Agitation: The solubility of a solute increases with an increase in agitation, such as stirring. This is because agitation increases the interaction between solute and solvent molecules, leading to faster dissolution.
- Presence of other solutes: The presence of other solutes can affect the solubility of a solute. In some cases, the presence of one solute can increase the solubility of another, while in other cases, it can decrease it.
