AgI, BaF_2, and AgBr are all sparingly soluble salts. Which of these salts will
ID: 959371 • Letter: A
Question
AgI, BaF_2, and AgBr are all sparingly soluble salts. Which of these salts will be more soluble in an acidic solution than in water? BaF_2 is the correct answer. The addition of H^+ ions (from a strong acid such as HCl is useful to aid the solubility of BaF_2 because the H^+ ions react with the F ions to form the weak acid HF. Because HF is a weak acid, it has a relatively small K_a. In other words, the equilibrium of the dissociation reaction HF H^+ + F^- lies to the left. Thus the F^- ions are effectively removed from solution, since they form HF with the H^+ ions. This pulls the equilibrium of the dissociation reaction of the sparingly soluble salt, BaF_2 Ba^2+ +2F^- to the right, increasing the solubility of BaF_2. As a chemist for an agricultural products company, you have just developed a new herbicide, "Herbigon," that you think has the potential to kill weeds effectively. A sparingly soluble salt, Herbigon is dissolved in 1 M acetic acid for technical reasons having to do with its production. You have determined that the solubility product K_sp of Herbigon is 8.90 times 10^-6. Although the formula of this new chemical is a trade secret, it can be revealed that the formula for Herbigon is X-acetate (XCH_3 COO, where "X" represents the top-secret caption of the salt). It is this caption that kills weeds. Since it is critical to have Herbigon dissolved (it won't kill weeds as a suspension), you are working on adjusting the pH so Herbigon will be soluble at the concentration needed to kill weeds. What pH must the solution have to yield a solution in which the concentration of X^+ is 5.50 times 10^-3 M ? The pK_a of acetic acid is 4.76.Explanation / Answer
In general, if the anion of a salt is the conjugate base of a weak acid, then the salt is more soluble in acidic solution. If it is the conjugate base of a strong acid, it is not more soluble in acid.
1) not more soluble, since I- is the conjugate base of the strong acid HI
2) more soluble, since BaF2 is the conjugate base of the weak acid 2F-
3) not more soluble, since Br- is the conjugate base of the strong acid HBr
b) pH = pKa - log( [ X+ ] / [acid] )
log (5.5x10^-3 M / 1 M )
pH = 4.76 - (-2.259)
pH = 7.019