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 8.60×106.

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 6.00×103M ? The pKa of acetic acid is 4.76.

Explanation / Answer

Ksp of herbigon = 8.60 X 10^-6 = [X+] [C2H3O2-]

Now as desired we need [X+] = 6.00×103M
So,
8.60 X 10^-6 = [X+] [C2H3O2-] = 6.00×103 X [C2H3O2-]
Hence,

[C2H3O2-] = 1.43 X 10^-3 M

we are given with pKa of acetic acid = 4.76

pKa = -logKa

So Ka = 1.74 X 10^-5

Ka = [H+] [ CH3COO-] / [CH3COOH]
Given: [CH3COOH] = 1 M

1.74 X 10^-5 = [H+] [1.43 X 10^-3] / 1

H+= 1.22 X 10^-2

pH = -log[H+] = 1.91

So the pH of solution should be = 1.91

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