Part B As a chemist for an agricultural products company, you have just develope
ID: 1021158 • Letter: P
Question
Part B
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 9.10×106.
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
XAc(s) + HAc(aq) X(aq) + H(aq) + Ac(aq)
Dissociation constant Ka = [H][Ac]/[HAc]
Ka = 10^(-4.76).
Ka = 1.74*10^-5.
1.74*10^-5 = x(x+0.00450)/(1-x)
1.74*10^-5= x2+ 0.00450x/(1-x)
(1-x)* 1.74*10^-5 = x2+ 0.00450x
1.74*10^-5 = x2+ 0.00450a -0.0000174x
x2 + 0.0044826 = 1.74*10^-5
x2 = 1.7355174 x10^-5
x= 0.004166 M = [H]
pH= log[H]
pH = log[0.004166]
pH= 2.38