Question

Brucine is a bitter alkaloid closely related to strychnine. It occurs in several plant species, the most well known being the Strychnos nux-vomica tree, found in South-East Asia. While brucine is related to strychnine, it is not as poisonous. Nevertheless, a human consuming over 2 milligrams of pure brucine will almost certainly suffer symptoms resembling strychnine poisoning. For medicinal purposes, brucine is primarily used in the regulation of high blood pressure and other comparatively benign cardiac ailments. Brucine (C₂₃H₂₆N₂O₄ ; mm = 394.46 g/mol; K_b = 1.9 x 10^-6) and strychnine (C₂₁H₂₂N₂O₂; mm = 334.41 g/mol; K_b = 1.8 x 10^-6) form crystalline solids upon reaction with hydrochloric acid (stomach acid). Both brucinium chloride (C₂₃H₂₇N₂O₄⁺ Cl^-; mm = 430.92 g/mol) and strychninium chloride (C₂₁H₂₃N₂O₂⁺ Cl^-; mm = 370.87 g/mol) are very soluble.

What is the molar concentration (M) of a solution of brucinium chloride that has a pH = 5.77?

Answer 1

Ans. Given, Kb of brucine = 1.9 x 10^-6

Now, pKb = - log (Kb) = -log (1.9 x 10^-6) = 5.72

# pOH of solution = 14.00 – pH = 14.00 - 5.77 = 8.23

# A solution of Brucine and brucinium chloride (when HCl is added to pure solution of Brucine) for a buffer of weak base and its conjugate acid.

Using Henderson-Hasselbalch equation for base-

            pOH = pKb + log ([BH⁺] / [B])

            Where,

                        B = Base = Brucine

                        BH⁺ = Conjugate acid = Brucinium chloride (BH⁺Cl^-)

            Putting the values in above equation-

            Or, 5.77 = 5.72 + log ([BH⁺] / [B])

            Or, 5.77 – 5.72 = log ([BH⁺] / [B])

            Or, ([BH⁺] / [B]) = antilog (0.05)

            Or, ([BH⁺] / [B]) = 1.122

            Or, [BH⁺] = 1.122 [B]

# To calculate molar concertation of BH⁺, the unknown [B] in above term “[BH⁺] = 1.122 [B]” must be provided.

# Please multiply [B] with 1.122 to get [BH⁺], if [B] in known. In this question, [B] is NOT known. Or, there is NOT information available to calculate [B].