Question

120 mg of morphine are dissolved in 20.0 mL water. This solution is titrated with 0.0225 M HCl.

Calculate the pH at the equivalence point of the titration

Answer 1

first calculate the concentration of morphine

M = (W /MW) x (1000 / V of solution in mL)

W = 120 mg = 0.12 g

MW = 285.34 g /mol

M = (0.12 / 285.34) x (1000 / 20)

M = 0.00042 x 50

M = 0.021

millimoles of morphine = 20 x 0.021 = 0.42

0.42 millimoles HCl must be added to reach equivalent point

0.42 = V x 0.0225

V = 18.7 mL

total volume = 18.7 + 20 = 38.7 mL

salt = 0.42 / 38.7 = 0.011 M

pOH = 1/2 [pKw + pKb + log C]

pKb of morphine = 5.79

pOH = 1/2 [14 + 5.79 + log 0.011]

poH = 1/2 [17.83]

pOH = 8.915

pH = 14 - 8.915

pH = 5.085