Calculate the pH

Question

Calculate the pH during the titration of 30.00 mL of 0.1000 M morphine with 0.1000 M HCl(aq) after 18 mL of the acid have been added. K_b of morphine = 1.6 x 10^-6.

Answer is 8.03

Answer 1

morphine + HCl ---> morphineH+ + Cl-

initial moles morphine = 0.1 M x 30 ml = 3 mmol

moles HCl added = 0.1 M x 18 ml = 1.8 mmol



[morphineH+] formed = 1.8 mmol/46 ml = 0.039 M

[morphine] remained = (3-1.8) mmol/46 ml = 0.026 M

pKa = -log(1 x 10^-14/1.8 x 10^-6) = 8.26

Using Hendersen-Hasselbalck equation,

pH = pKa + log(morphine/morphineH+)

    = 8.26 + log(0.026/0.039)

    = 8.08