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+H2O⇌C5H5NH++OH−

The pKb of pyridine is 8.75. What is the pH of a 0.185 M solution of pyridine? (Assume that the temperature is 25 ∘C.)

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:

C6H5COOH⇌C6H5COO− + 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?

Answer 1

? (shs Nht + oH A) CS MEN + H2O 0.185M (initial) 0.1854 0.185. 0.18582 laurel (& is very small for weak bad) 0.185 (1-0) Kb = (0.1856) (0.1856) 0.185 (1-a) Given : PKb = 8.75 Since; PKb = -log, Kb K6= 10-pki * 10 Ab = .-8.75 10 = 1.78x10-9 0.185x² = 1.78X10 D: 185 M x = 1.788 1009 = 0.96 x 1018 d = (0.46X108) ", 0.98x10-4 Cor? ) = 0-1858 = 0.185 *0.48 x10 1.8x 10m Since; POH = -logo [on-) = -logno (1-8X105) – 4.74 As we know: PH+POH = 14 i phe 14-Pon 14-4.74 = 9.26 Hence; Ph of 0.185 solution of pyridine is 9.26

0.620 0.62x B) GH5 ook ons loo + H+ 0.62 m 0 0 (initial) 0.62 (1-0) ka = 10.620) (0.62d) 0.620² 0.6211-«) 1-821 (& is very small Giren: Pka = 4.2 ыл was ud!) Since; Pka = -logpo ka ka = 10-tka = 104 6.31x10-5 0.6287 = 60 31x10-5 at 6:31 X 100% = 1.02x10-4 d = (1.02 x 104) 101x102 - 0.62 % of molecules inized = xx 100% = 101 x 102x100 % = 1.01%