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 8.30×10−6.

Although the formula of this new chemical is a trade secret, it can be revealed that the formula for Herbigon is X-acetate (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 1.50×10−3 M ? The pKa of acetic acid is 4.76. Express your answer numericall

Answer 1

ans)

from above data that

Ksp = [X+][C2H3O2-] = 8.3 * 10^-6

Substituting in the desired value for [X+]

8.3 * 10^-6 = (1.5*10^-3)x

x, or acetate concentration, is .005533

Now, we calculate the Ka of acetic acid:

pKa = -logKa

4.76 = -logKa
Ka = 1.738*10^-5

We want the concentration of acetate to be .002686, so we plug that into our acid dissociation constant expression:
Ka = [H+][C2H3O2-]/[HC2H3O2]

Ka = x(.005533)/1

1.738*10^-5=x(.005533)/1

x = 0.00314 = [H+]

pH = -log[H+]
pH = -log(.003141)
pH = 2.50