Question

Quinine (C₂₀H₂₄N₂O₂) is the most important alkaloid derived from cinchona bark. It is used as an antimalarial drug. For quinine pKb₁ = 5.1 and pKb₂ = 9.7 (pKb₃ = - log Kb). Only 1.0 g of quinine will dissolve in 1900.0 mL of solution. Calculate the pH of a saturated aqueous solution of quinine. Consider only the reaction

Q + H₂O ⇋ QH⁺ + OH⁻

described by pKb₁, where Q = quinine.

Answer 1

To calculate the pH of a saturated aqueous solution of quinine, we can use the formula pH = pKb + log(QH+ / Q), where Q is the concentration of quinine and QH+ is the concentration of its conjugate acid.

Step-by-step explanation:

The pH of a saturated aqueous solution of quinine can be calculated using the formula:

pH = pKb + log(QH+ / Q)

Where Q is the concentration of quinine and QH+ is the concentration of its conjugate acid.

In this case, we are given that 1.0 g of quinine dissolves in 1900.0 mL of solution. To find the concentration of quinine in mol/L, we can use its molar mass:

1.0 g / (molar mass of quinine) = moles of quinine

moles of quinine / 1.9 L = concentration of quinine

Next, we can use the pKb value (5.1) to find the concentration of QH+ using the equation:

pKb + log(QH+ / Q) = 14

Solving for QH+, we find:

QH+ = 10(14 - 5.1) * Q

Finally, we can substitute the values into the equation to find the pH.