Calculate the molar solubility of Mg(OH)2 in the following solvents:
(i) Pure Water
(ii) 8.60×10−2M MgCl2
(iii) 3.70×10−2M KOH(aq)
How should i solve this problem?
In pure H2O:
.........Mg(OH)2(s) ==> Mg^2+ + 2OH^-
I.........solid.......0...........0
C..........-x.........x....,,,....2x
E.........solid-x.....x...........2x
Substitute the E line into Ksp expression and solve for x = solubility Mg(OH)2 in mols/L.
NOTE: Mg(OH)2(s) is NOT included in the Ksp expression; therefore, solid-x will not appear.
What is the value of Ksp?
To calculate the molar solubility of Mg(OH)2 in different solvents, you can use the concept of solubility product constant (Ksp). The Ksp is a measure of the extent to which a compound dissolves in a solvent.
In order to determine the molar solubility, you need to set up an equilibrium expression using the given Ksp values and the stoichiometry of the balanced chemical equation.
Here's the step-by-step approach to solve the problem:
(i) Pure Water:
1. Write the balanced chemical equation for the dissolution of Mg(OH)2 in water:
Mg(OH)2(s) ⇌ Mg²⁺(aq) + 2OH⁻(aq)
2. Write the expression for the Ksp of Mg(OH)2:
Ksp = [Mg²⁺][OH⁻]²
3. Since Mg(OH)2 is a sparingly soluble salt, it undergoes limited dissociation in water. Assume the molar solubility of Mg(OH)2 to be x.
4. Substitute the values into the equilibrium expression:
Ksp = (x)(2x)²
5. Solve for x, the molar solubility of Mg(OH)2.
(ii) 8.60×10−2M MgCl2:
1. Determine the initial concentrations of Mg²⁺ and OH⁻ ions in the solution based on the concentration of MgCl2.
2. As magnesium ions (Mg²⁺) are already present in the solution, consider x as the additional molar solubility of Mg(OH)2.
3. Write the expression for the Ksp of Mg(OH)2 as before:
Ksp = (x + [Mg²⁺])(2x)
4. Since the concentration of Mg²⁺ is given as 8.60×10−2M, substitute the value into the equation.
5. Solve for x, the additional molar solubility of Mg(OH)2.
(iii) 3.70×10−2M KOH(aq):
1. Determine the initial concentrations of Mg²⁺ and OH⁻ ions in the solution based on the concentration of KOH.
2. Since potassium ions (K⁺) are already present in the solution, consider x as the additional molar solubility of Mg(OH)2.
3. Write the expression for the Ksp of Mg(OH)2 as before:
Ksp = (x)(2x + [OH⁻])
4. Since the concentration of OH⁻ is given as 3.70×10−2M, substitute the value into the equation.
5. Solve for x, the additional molar solubility of Mg(OH)2.
By following these steps, you can calculate the molar solubility of Mg(OH)2 in each given solvent.
I don't know. Look it up in your text/notes or on the web.
By the way, part 2 and 3 of the problem are common ion problems and each will decrease the solubility; i.e., solubility of parts 2 and 3 will be less than the solubility of part 1.