Secobarbital [C_12 H_15N_2 O_2] is usually delivered as the sodium salt (NaC_12,

Question

Secobarbital [C_12 H_15N_2 O_2] is usually delivered as the sodium salt (NaC_12, H_12, N_2 O_2) and is often marketed under the brand name Seconal). It is a barbiturate derivative drug that possesses anaesthetic, anticonvulsant, anxiolytic, sedative, and hypnotic properties. It is usually delivered in capsules containing 100 mg of NaC_12 H_12 N_12 O_3 Determine the of the pH of a solution made by dissolving one capsule in 400 ml of solution (about a cup of water) if the pKa of secobarbital is 7.8.

Explanation / Answer

pKa = 7.8

pKb = 14 - 7.8 = 6.2

pKb = -log(Kb)

Kb = 6.31 x 10^-7

initial [NaC12H17N2O3] = 0.1 g/238.3 g/mol x 0.4 L = 1.05 x 10^-3 M

NaC12H17N2O3 --> Na+ + C12H17N2O3-

C12H17N2O3- + H2O <==> C12H18N2O3 + OH-

let x amount has reacted

Kb = [C12H18N2O3][OH-]/[C12H17N2O3-]

6.31 x 10^-7 = x^2/1.05 x 10^-3

x = [OH-] = 2.57 x 10^-5 M

pOH = -log[OH-] = 4.59

thus,

pH = 14 - pOH = 9.41

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