Which of the following salts would show an appreciable pH-dependent solubility?
a. AgCl
b. Mg(OH)2
c. BaF
d. both b and c
e. all of the above
I put c as my answer (not sure if its right, but can you explain how you would solve this)
The correct answer is d; i.e., both b and c.
You look at the solubility as well as if the H^+ reacts with cation or anion to produce a weak acid. If so, then it is pH dependent.
AgCl, for example, ==> Ag^+ + Cl^-.
Adding H^+ might try to form HCl but that is not a weak acid; therefore, changing the pH is not a problem. (It is generally understood that the solution cannot become too basic or pptn of Ag2O will occur.)
Mg(OH)2 obviously will be affected by the pH because of the OH^- ion and H^+ + OH^- => H2O.
BaF2 (it's BaF2 and not BaF) is affected because of the possible formation of HF, a weak acid.
BaF2(s) ==> Ba^+2 + 2F^-
Note that F^- + H^+ ==> HF
Thus as the solution is made more acidic, the H^+ removes the F^- (actually both remove each other). Remember Le Chatelier's Principle: when we remove the F^- (by adding more H^+), the reaction (the solubility) shifts to the right which means that more of the BaF2(s) dissolves to form more Ba^+ and 2F^-. Therefore, the more acid we add, the more F^- is removed, and the more the reaction shifts to the right, and the more soluble BaF2 is in the solution.
To determine which of the provided salts would show a pH-dependent solubility, we need to consider the nature of the cation and anion in each case.
a. AgCl: Silver chloride (AgCl) is a soluble salt with a very low solubility product (Ksp). It does not undergo significant hydrolysis or dissociation in water. The solubility of AgCl remains relatively constant regardless of the pH. Therefore, AgCl does not exhibit appreciable pH-dependent solubility.
b. Mg(OH)2: Magnesium hydroxide (Mg(OH)2) is a sparingly soluble salt. It has a higher solubility product (Ksp) than AgCl. In water, magnesium hydroxide undergoes a dissociation reaction: Mg(OH)2 ⇔ Mg2+ + 2OH-. The hydroxide ion (OH-) concentration depends on the pH of the solution. As the pH increases, the hydroxide ion concentration also increases, leading to an increased solubility of Mg(OH)2. Therefore, Mg(OH)2 does show appreciable pH-dependent solubility.
c. BaF: Barium fluoride (BaF) is a highly insoluble salt, and its solubility product (Ksp) is very low. Like AgCl, BaF does not undergo appreciable hydrolysis or dissociation in water. Consequently, the solubility of BaF remains relatively constant regardless of the pH. Thus, BaF does not exhibit appreciable pH-dependent solubility.
d. both b and c: As we discussed above, only magnesium hydroxide (Mg(OH)2) shows appreciable pH-dependent solubility. Therefore, option d. (both b and c) is incorrect.
e. all of the above: Since only option b. (Mg(OH)2) satisfies the condition of pH-dependent solubility, option e. (all of the above) is also incorrect.
Therefore, the correct answer is c. BaF.