Question

Amphetamine (C9H13N) is a weak base with a pKb of 4.2. Calculate the pH of a solution containing an amphetamine concentration of 215 mg/L .

Answer 1

pH of weak base solution

[amphetamine] = 0.215 g/L x 135.21 g/mol

                         = 0.0016 M

let x amount of base reacts in solution to form salt

then,

Kb = [amphetamineH+][OH-]/[amphetamine]

pKb = -logKb] = 4.2

So,

Kb = 6.31 x 10^-5

thus,

6.31 x 10^-5 = x^2/0.0016

x = [OH-] = 3.18 x 10^-4 M

pOH = -log[OH-] = 3.50

So pH of the solution would be,

pH = 14 - pOH = 10.5