As a chemist for an agricultural products company, you have just developed a new
ID: 881093 • 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 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.
Express your answer numerically. Answer is not 2.46!
Explanation / Answer
Given:
Concentration of X+ = 1.50E-3 M
We know XCH3COO = 1.50 E-3 (same concentration as metal has )
Lets show ICE chart
CH3COO-(aq) + H2O (l) ------ > CH3COOH (aq) + OH-
I 1.50 E-3 0 0
C -x +x +x
E (1.50E-3-x) x x
Kb = [CH3COOH ] [OH-] / [CH3COO-]
Kb = 1.0E-14/ ka
Ka = antilog (- pka) = Antilog (-4.76) = 1.77 E-5
Kb = 1.4E-14/ 1.77E-5
= 5.65 E-10
We use this value to get OH- concentration
5.65 E-10 = x2/ ( 1.50E-3 – x)
Value of x is very small so we can use following approximation
1.50E-3 – x = 1.50E-3
5.65 E-10 = x2/ 1.50E-3
x = 9.20 E-7 = [OH-]
pOH = -log [OH-]
= 6.036
pH = 14- 6.036 = 7.96
pH of the solution = 7.96
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