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 Macetic acid for technical reasons having to do with its production. You have determined that the solubility product Ksp of Herbigon is 8.90×106.

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 3.00×103M ? The pKa of acetic acid is 4.76.

Express your answer numerically.

Explanation / Answer

this is a salt

A- is formed

the next equilibrium is formed, the conjugate acid and water

A- + H2O <-> HA + OH-

Kb by definition since it is an base:

Kb = [HA ][OH-]/[A-]

Ka can be calcualted as follows:

Kb = Kw/Ka = (10^-14)/(1.8*10^-5) = 5.55*10^-10

and;

assume x = [OH-]

[OH-] = x= [HA] due to equilibrium

[A-] = M-x = 0.42-x

Kb = [HA ][OH-]/[A-]

5.55*10^-10 = x*x/(3*10^-3-x)

solve for x with quadratic equation

x = OH- =1.29*10^-6

[OH-]  = 1.29*10^-6 M

pOH = -log(OH-) = -log(1.29*10^-6) = 5.89

pH = 14-5.89= 8.11

pH = 8.11

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