Molar Solubility, Solubility Product Constant, and Predicting Precipitation
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Need help preparing for the General Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about molar solubility, solubility product constant, and predicting precipitation of solutions. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!
Solubility is evident in your day-to-day life. For instance, consider a glass of drinking water. If you added table salt into the water, it would dissolve into the water and become unnoticeable. However, after a certain amount of table salt is added, you’ll notice it begin to pile up at the bottom of the glass. For a solute in a given solvent, the maximum amount of solute that can dissolve is defined as the molar solubility of that solute.
Molar solubility
Molar solubility (S) is defined as the maximum concentration of a solute that can dissolve in solution. Given this propensity to hold a maximum amount of solute, we can consider three different types of solutions, where the current concentration of dissolved solute is less than, equal to, or greater than S: unsaturated solutions, saturated solutions, and supersaturated solutions.
1. Unsaturated solutions feature a concentration of dissolved solute less than the maximum molar solubility. In an unsaturated solution, if you were to add additional solutes to solution, those solutes would dissolve.
[solute] less than S
2. Saturated solutions feature a concentration of solute equal to the maximum allowable concentration (S). In a saturated solution, since you've already dissolved the maximum amount of solute possible, if you were to add more solutes the solutes would not dissolve. Instead, they would precipitate. Precipitation is the formation of solid molecules by solute particles, as opposed to those solute particles which dissolve in solution.
[solute] = S
3. A supersaturated solution is one where the solute concentration is greater than the maximum possible molar solubility. This paradoxical type of solution doesn't happen under typical situations, and must be created experimentally. A supersaturated solution can be created by first heating a solution to extremely high temperatures, allowing solute molecules to dissolve, and finally rapidly cooling the solution without disturbing it. In some instances, it’s possible that number of molecules remaining suspended in solution will exceed the molar solubility.
[solute] greater than S
Solubility Product Constant
The solubility product constant is another measure of solubility, referring specifically to electrolytes. Recall that electrolytes are composed of cations and anions. In solution, electrolytes will dissociate into their constituent ions. The solubility product constant is the equilibrium constant expression for the dissociation of ions in solution. Consider the electrolyte MgCl2 dissolved in solution. It’s solubility product constant expression (it’s Ksp), the product of its ion pairs, would be written as:
One task you may be assigned by the MCAT is to relate the solubility product constant with the molar solubility. At equilibrium, a solution is considered saturated. When a solution is saturated, a concentration of MgCl2 equal to the molar solubility (S) is dissolved in solution. Note that for every amount S of MgCl2 dissolved in solution, one part Mg and two parts Cl are produced. Plugging these values into Ksp, we find that the Ksp would for a saturated solution of MgCl2 would be equal to 4S3.
Predicting Precipitation
We know that the expression for calculating the equilibrium concentration of ionic compounds in solution is given by Ksp. If the product of ions in solution exceeded this value, the solution would be supersaturated. In that case, excess ions in the solution would precipitate. Recall that the reactant quotient (Q) is the current ratio of products to reactants in solution. Given a solution with some current value of Q and an equilibrium expression Ksp, there are three cases we can consider.
1. When Q less than Ksp, the current product of ions in solution is less than the equilibrium solubility product constant. Therefore, the solution is unsaturated. Any solute added to solution will dissolve as the system proceeds toward equilibrium.
2. When Q = Ksp, the solution is at equilibrium and is saturated. Any excess solute added will precipitate out of solution.
3. When Q greater than Ksp, the solution is supersaturated. This solution is highly unstable and excess solute will almost certainly precipitate out of solution.
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Solubility is evident in your day-to-day life. For instance, consider a glass of drinking water. If you added table salt into the water, it would dissolve into the water and become unnoticeable. However, after a certain amount of table salt is added, you’ll notice it begin to pile up at the bottom of the glass. For a solute in a given solvent, the maximum amount of solute that can dissolve is defined as the molar solubility of that solute.
Molar solubility
Molar solubility (S) is defined as the maximum concentration of a solute that can dissolve in solution. Given this propensity to hold a maximum amount of solute, we can consider three different types of solutions, where the current concentration of dissolved solute is less than, equal to, or greater than S: unsaturated solutions, saturated solutions, and supersaturated solutions.
1. Unsaturated solutions feature a concentration of dissolved solute less than the maximum molar solubility. In an unsaturated solution, if you were to add additional solutes to solution, those solutes would dissolve.
[solute] less than S
2. Saturated solutions feature a concentration of solute equal to the maximum allowable concentration (S). In a saturated solution, since you've already dissolved the maximum amount of solute possible, if you were to add more solutes the solutes would not dissolve. Instead, they would precipitate. Precipitation is the formation of solid molecules by solute particles, as opposed to those solute particles which dissolve in solution.
[solute] = S
3. A supersaturated solution is one where the solute concentration is greater than the maximum possible molar solubility. This paradoxical type of solution doesn't happen under typical situations, and must be created experimentally. A supersaturated solution can be created by first heating a solution to extremely high temperatures, allowing solute molecules to dissolve, and finally rapidly cooling the solution without disturbing it. In some instances, it’s possible that number of molecules remaining suspended in solution will exceed the molar solubility.
[solute] greater than S
Solubility Product Constant
The solubility product constant is another measure of solubility, referring specifically to electrolytes. Recall that electrolytes are composed of cations and anions. In solution, electrolytes will dissociate into their constituent ions. The solubility product constant is the equilibrium constant expression for the dissociation of ions in solution. Consider the electrolyte MgCl2 dissolved in solution. It’s solubility product constant expression (it’s Ksp), the product of its ion pairs, would be written as:
One task you may be assigned by the MCAT is to relate the solubility product constant with the molar solubility. At equilibrium, a solution is considered saturated. When a solution is saturated, a concentration of MgCl2 equal to the molar solubility (S) is dissolved in solution. Note that for every amount S of MgCl2 dissolved in solution, one part Mg and two parts Cl are produced. Plugging these values into Ksp, we find that the Ksp would for a saturated solution of MgCl2 would be equal to 4S3.
Predicting Precipitation
We know that the expression for calculating the equilibrium concentration of ionic compounds in solution is given by Ksp. If the product of ions in solution exceeded this value, the solution would be supersaturated. In that case, excess ions in the solution would precipitate. Recall that the reactant quotient (Q) is the current ratio of products to reactants in solution. Given a solution with some current value of Q and an equilibrium expression Ksp, there are three cases we can consider.
1. When Q less than Ksp, the current product of ions in solution is less than the equilibrium solubility product constant. Therefore, the solution is unsaturated. Any solute added to solution will dissolve as the system proceeds toward equilibrium.
2. When Q = Ksp, the solution is at equilibrium and is saturated. Any excess solute added will precipitate out of solution.
3. When Q greater than Ksp, the solution is supersaturated. This solution is highly unstable and excess solute will almost certainly precipitate out of solution.
MEDSCHOOLCOACH
To watch more MCAT video tutorials like this and have access to study scheduling, progress tracking, flashcard and question bank, download MCAT Prep by MedSchoolCoach
IOS Link: https://play.google.com/store/apps/de...
Apple Link: https://apps.apple.com/us/app/mcat-pr...
#medschoolcoach #MCATprep #MCATstudytools
4 سال پیش
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