All parts of the hemlock plant contain the alkaloid coniine. Coniine (C_8H_17N)

Question

All parts of the hemlock plant contain the alkaloid coniine. Coniine (C_8H_17N) is an alkaline liquid with a pK_b of 3.1, and a density of 0.844 g/ml. Ingestion of this substance may cause weakness, drowsiness, nausea, vomiting, labored respiration, paralysis and death. Explain how you would prepare a 250 ml solution of coniine that has a pH of 12.0. Calculate the pH of a buffer system made by dissolving 2.8 grams of sodium nitrite (NaNO_2) in 100 ml of solution and mixing that with 150 ml of a 0.35 M HNO_2 solution if the Ka of nitrous acid is 3.98 times 10^-4

Explanation / Answer

8. Coniine pKb = 3.1

pKb = -log[Kb]

Kb = 7.94 x 10^-4

pH = 12

pOH = 14 - pH = 2

pOH = -log[OH-]

[OH-] = 1 x 10^-2 M

Base picks up proton,

C8H17N + H2O <==> C8H17NH+ + OH-

Kb = [C8H17NH+][OH-]/[C8H17N]

7.94 x 10^-4 = (0.01)^2/[C8H17N + 0.01]

[C8H17N] = 0.136 M

So to prepare 250 ml coniine solution of pH 12,

amount of coniine to be taken = 0.136 M x 0.250 L x 127.23 g/mol/0.844 g/ml

                                                = 5.13 ml

So, 5.13 ml of coniine in 250 ml would give a solution of pH 12

9. molarity of NaNO2 in buffer solution (base) = 2.8 g/69 g/mol x 0.250 L = 0.16 M

molarity of HNO2 in buffer solution (acid) = 0.35 M x 0.150 L/0.250 L = 0.21 M

pKa = -logKa = 3.4

pH = pKa + log(base/acid)

     = 3.4 + log(0.16/0.21)

     = 3.282

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