Part A Pyridine is a weak base that is used in the manufacture of pesticides and

Question

Part A

Pyridine is a weak base that is used in the manufacture of pesticides and plastic resins. It is also a component of cigarette smoke. Pyridine ionizes in water as follows:

C5H5N+H2OC5H5NH++OH

The pKb of pyridine is 8.75. What is the pH of a 0.220 M solution of pyridine? (Assume that the temperature is 25 C.)

Express the pH numerically to two decimal places.

Part B

Benzoic acid is a weak acid that has antimicrobial properties. Its sodium salt, sodium benzoate, is a preservative found in foods, medications, and personal hygiene products. Benzoic acid ionizes in water:

C6H5COOHC6H5COO+H+

The pKa of this reaction is 4.2. In a 0.62 M solution of benzoic acid, what percentage of the molecules are ionized?

Express your answer to two significant figures and include the appropriate units.

Explanation / Answer

This is a base in water so, let the base be "B" and HB+ the protonated base "HB+"

there are free OH- ions so, expect a basic pH

B + H2O <-> HB+ + OH-

The equilibirum Kb:

Kb = [HB+][OH-]/[B]

let "x" be OH- in solution

in equilibrium due to sotichiometry:

[HB+]= x= [OH-]

Acocunt for the dissolved base in solution vs. not in solution:

[B] = 0.05-x

Substitute in Kb

Kb = 10^-pKb = 10^-8.75 =1.77827*10^-9

Kb = [HB+][OH-]/[B]

1.77827*10^-9= x*x/(0.22-x)

Solve for x, using quadratic equation

x = OH- = 1.978*10^-5

pOH = -log(OH-) = -log( 1.978*10^-5) = 4.703

pH = 14-pOH = 14-4.703

pH = 9.30

B)

By definition, in equilibrium:

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

Then

Assume that [H+] = [A-] = x … due to stoichiometry (i.e. 1 mol of H+ per each mol of A-)

Then, in equilibrium [HA] = M – x (i.e. the original concentration minus the acid in equilibrium)

Substitute

Ka = (x*x)/(M-x)

Ka = 10^-pKa = 10^-4.2

6.309*10^-5 = x*x/(0.62-x)

solve for x (quadratic equation)

x = [H+] = 0.006223 M

[H+] = [A-] = x = 0.006223

The ionization:

% ionization = [H+] / M * 100% = 0.006223 /(0.62) * 100% = 1.003709 %

2 sig fig --< 1.0 %

Related Questions

Navigate

• Browse (All)
• Browse P
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.