As a chemist (or an agricultural products company, you have just developed a new

Question

As a chemist (or 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 mol L^-1 acid for technical reasons having to do with its production. You have determined that the solubility product K_sp of Herbigon is 8.70 times 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 (XCH_3COO, whore "X" represents the top-secret cation of the salt). It is this cation that kills weeds Since it is critical to have Herobigon dissolved (it won't kill weeds as a suspension), you are working on adjusting the pH so Horbtigon will be soluble at the concentration to kill weeds What pH must the solution have to yield a solution in which the concentration of X is 4.50 times 10^-3 mol L^-1? The pK_a of acetic acid is 4.76.

Explanation / Answer

Ksp = 8.70e-6 = [X+][Ch3COO-]
we know what [X+] we need, 6.00e-3M. so we can solve for[CH3COO-]:
[CH3COO-]=8.70e-6/[X+] = 1.933e-3M
pKa of acetic acid is 4.76=-log10Ka, soKa=10-4.76
Ka=[H+][CH3COO-]/[CH3COOH], we know that herbigon is dissolved in1M acetic acid solution. This means that [CH3COOH]=1M.
so,
10-4.76=[H+]*1.933e-3M/1M,[H+]=10-4.76*1M/1.933e-3M=8.73e-3
pH=-log10([H+]/1mol/L)=-log10(8.73*10^-3) =2.05

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