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 8.00×10⁻⁶.

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 4.50×10⁻³M ? The pKa of acetic acid is 4.76.

Answer 1

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