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 9.50×10⁻⁶.

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 1.50×10⁻³M ? The pKa of acetic acid is 4.76.

Express your answer numerically. Answer is not 2.46!

Answer 1

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