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 9.50×106.

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 1.50×103M ? The pKa of acetic acid is 4.76.

Express your answer numerically.

Explanation / Answer

Ksp = 9.5 x 10^-6

9.5 x 10^-6 = [CH3COO-][X+]

                   = [CH3COO-](1.5 x 10^-3)

[CH3COO-] = 6.33 x 10^-3 M

CH3COO- + H2O <==> CH3COOH + OH-

let x amount of acetate has reacted then,

Kb = [CH3COOH][OH-]/[CH3COO-]

pKa = 4.76 = -log[Ka]

Ka = 1.74 x 10^-5

Kb = Kw/Ka = 1 x 10^-14/1.74 x 10^-5 = 5.75 x 10^-10

5.75 x 10^-10 = x2/6.33 x 10^-3

x = [OH-] = 1.91 x 10^-6 M

pOH = -log[OH-] = 5.72

pH = 14 - pOH = 8.28

So we need a pH of 8.28

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