Question

A 1.00 g pill contains morphine (M = 285.34), a monobasic compound whose conjugate acid has pKa = 8.21, in addition to an unknown amount of inert material. To analyze the morphine content of the pill, it is dissolved in 50.0 mL 0.1000 M aqueous HCl and the unreacted acid in the resulting solution back-titrated with 0.1000 M NaOH. Which of the following statements about this experiment are correct?

I. If the pill contains only morphine, the titration will require 35.0 mL NaOH solution to reach the endpoint.

II. Methyl red, which changes color from red to yellow between pH = 4.4 and pH = 6.2, would be a suitable indicator for this titration.

(A) I only (B) II only (C) Both I and II (D) Neither I nor II

The correct answer is B but I dont understand why. Thank you.

Answer 1

Solution:

The pKa of conjugate acid of morphine is 8.21, suggesting that the conjugate acid is weakly acidic, therefore, according to conjugate acid-base concept, it is confirmed that the morphine is working as a strong base. The acidic and basic nature of morphine depends upon the presence of two different groups in morphine. The phenolic group in morphine makes it slightly acidic, while amine group makes it strongly basic.

Thus, if only morphine is present in pill, then titration with NaOH does not provide end point due to its basic character.

Since, methyl red gives color in acidic medium ( pH 4.4 - 6.2), therefore used as an indicator to measure the color change during titration.

Hence, only II statement is correct.