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 K_sp of Herbigon is 9.50x10^-6. Although the formula of this new chemical is a trade secret, it can be revealed that the formula for Herbigon is X-acetate (XCH_3 COO, 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 5.00x 10^-3 M? The pK_a of acetic acid is 4.76.

Explanation / Answer

M = 5*10^-3

pKa = 4.76

Ka = 10^-pKa = 10^-4.76 = 1.7378*10^-5

Ka = [H+][A-]/[HA]

[HA] = M-x

substitute

1.7378*10^-5= X*x/(5*10^-3x)

x = 2.864*10^-4

pH= -log(2.864*10^-4)

pH = 3.54302

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