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 1.00×103M ? The pKa of acetic acid is 4.76.

Explanation / Answer

Ksp = 9.5*10^-6

pH for:

X+ = 10^-3 M

givne pKa = 4.76

so...

XCH3COO <-> X + + CH3COO-

Ksp = [X+][CH3COO-]

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

if [X+] = 10^-3 then

[CH3COO-] = (9.5*10^-6)/(10^-3) = 0.0095 M

so..

from the acetate equilibrium

HCH3COO <-> H+ +CH3COO-

Ka = [H+][CH3COO-]/[HCH3COO]

10^-4.76 = [H+]*0.0095 / (1-0.0095 )

[H+] = (10^-4.76) / (0.0095 ) * (1-0.0095 )

[H*] = 0.0018118

H+ = -log(0.0018118) = 2.7418

the pH must be at least 2.7418

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