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 .
Express your answer to two decimal places.

Answer 1

Weak base: [OH⁻] = √Kb.C

pKb = 4.2

`\tt Kb=10^(-4.2)`

c = concentration

MM Amphetamine (C9H13N) = 135.21 g/mol

c = 215 mg/L = (0.215 g : 135,21 g/mol) / L = 0.00159 mol/L = 1.59 x 10⁻³ mol/L

`\tt [OH^-]=\sqrt{10^(-4.2)* 1.59* 10^(-3)}=3.17* 10^(-4)`

pOH = 4 - log 3.17

pH = 14 - (4 - log 3.17)

pH = 10 + log 3.17 = 10.50