As a chemist for an agricultural products company, you have just developed a new
ID: 880961 • Letter: A
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 8.00×106.
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 4.50×103M ? The pKa of acetic acid is 4.76.
Explanation / Answer
We have to find pH of acetic acid
We know [XCH3COO] = [X+] = [CH3COO- ] ….salt dissociates completely
We show ICE chart
CH3COO- (aq) + H2O (l) ---- > CH3COOH (aq) + OH- (aq)
I 4.50E-3 0 0
C -x +x +x
E (4.50E-3-x) x x
We use kb
Kb = 1.0E-14/ ka ( acetic acid)
Ka = antilog (-pka) = 1.74 E-5
Lets find kb
Kb = 1.0E-14/ 1.74 E-5
= 5.75E-10
Kb expression
Kb = [CH3COOH][OH-]/[CH3COO-]
5.75 E-10 = x2/ (4.50E-3 –x)
Value of kb is very small ; 4.50E-3 -x = 4.50E-3
5.75 E-10 = x2/ 4.50E-3
x = 1.61 E-6
x = [OH-] = 1.61 E-6
pOH = -log [OH-] = -log ( 1.61E-6)
= 5.79
pH = 14 – pOH = 14 – 5.79 = 8.21
pH = 8.21
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