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 5.00×103 M ? The pKa of acetic acid is 4.76.

Explanation / Answer

CH3COOX <==> CH3COO- + X+

[X+] = [CH3COO-] = 5 x 10^-3 M

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

pKa = 4.76

pKb = 14 - 4.76 = 9.24

Kb = 5.75 x 10^-10 = x^2/5.0 x 10^-3

x = [OH-] = 1.69 x 10^-6 M

pOH = -log[OH-] = 5.77

pH = 14 - pOH = 8.23

So the pH of solution needed would be 8.23

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