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.

Explanation / Answer

The salt of herbigon with acetic acid will be dissociated as

CH3COO-X+   ---> CH3COO-   + X+

And CH3COOH --> CH3COO- + H+

We will use Hendersen Equation to solve the oncentration of salt

pH = pK + log [ Salt ] / [acid ]
pH = 4.76 + log [CH3COO-] / [CH3COOH ]
XCH3COO >> X+ + CH3COO-
So [X+] = [CH3COO-] =1.5 x 10^-3 = 0.0015 M
pH = 4.76 + log 0.0015 / [1]

pH = 4.76 + (-2.82) = 1.94

so pH should be 1.94

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