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 8.40×10?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 3.50×10?3 M ? The pKa of acetic acid is 4.76.

Explanation / Answer

XCH3COO <---> X+ + CH3COO-

the solubility product constant is given by

Ksp = [X+] [CH3COO-]

8.4 x 10-6 = [3.5 x 10-3] [CH3COO-]

[CH3COO-] = 2.4 x 10-3 M

now

CH3COOH (aq) ---> H+ + CH3COO-

according to hasselbalch hendersen equation

pH = pKa + log [CH3COO- / CH3COOH]

pH = 4.76 + log [0.0024 / 1]

pH = 2.14

so

the pH of the solution should be 2.14

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