Question

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 Ksp of Herbigon is 8.20x10-6 Although the formula of this new chemical is a trade secret, it can be revealed that the formula for Herbigon is X-acetate (XCH3COO, where "X" represents the top-secret cation of the salt). It is this cation 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 4.50x10-3 M? The PK, of acetic acid is 4.76. Express your answer numerically. View Available Hint(s) ► Hint 1. How to approach the problem Hint 2. Determine the maximum [Ac] Hint 3. Determine [H+] VO AQ * O O ? pH = Submit Request Answer wide Feedback Next >

Answer 1

The amount of acetate dissociating from the compound

Ksp = [ X+] [ C2H3O2-] = 8.20*10^-6

8.20*10^-6 = ( 4.50*10^-3) *   [ C2H3O2-]

[ C2H3O2-] = ( 8.20*10^-6 ) / ( 4.50*10^-3) =0.0018 M

pKa = -logKa

Ka = 10^-pKa = 10^-4.76

Ka = 1.738*10^-5

substituting in the equilibrium expression

Ka = [ H+] [ C2H3O2-] / [ HC2H3O2]

1.738*10^-5 = [ H+] ( 0.0018) / 1

[H+] = 1.738*10^-5 / 0.0018

[ H+] = 0.00965

pH = -log(H+)

pH = -log(0.00965)

pH = 2.01