Question

At 25 °C the value of Kb for codeine, a pain-killing drug, is 1.63 10-6. Calculate the pH of a 0.0115 M solution of codeine in water

Answer 1

codeine is a week base iw ill not dissociate completely

write equation first

Codine + H2O <------> CodineH+ + OH-

intial concentration 0.0155 0 0

after diaaociated x -x -x

concentration at equilibrium 0.0155 - x -x -x

now write the expression for the dissociation constant

Kb = 1.63 10^-6 = [CodineH+] [OH-] / [Codine]

1.63 10^-6 = [x] [x] / [0.0155 - x]

1.63 10^-6 = x² / 0.0155 -x

x² + 1.63 10^-6 x - 0.0252 x 10^-6 = 0

solve the quardratic equation

x = 0.000157 M = concentration of OH-

now

pOH = -log [OH-]

pOH = -log [0.000157]

pOH = 3.80

now use the fomula

pH + pOH = 14

pH = 14-3.80

pH = 10.2