As a chemist for an agricultural products company, you have just developed a new

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 8.10×106. 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 2.50×103 M ? The pKa of acetic acid is 4.76.

Explanation / Answer

Ksp = [X+][C2H3O2-] = 8.10*10^-6

Substituting in the desired value for [X+]

8.10*10^-6 = (2.5*10^-3)x

x, or acetate concentration, is 0.00324

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 0.00324 , so we plug that into our acid dissociation constant expression:
Ka = [H+][C2H3O2-]/[HC2H3O2]
Ka = x(0.00324)/1

x = .00536= [H+]

pH = -log[H+]
pH = -log(.00536)
pH = 2.27

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