Answer to Question #108483 in General Chemistry for Beverlie

Question

1. Pyridine (C5H5N) is a particularly foul-smelling substance used in manufacturing pesticides and plastic resins. Calculate the pH of a 0.125 M solution of pyridine (Kb = 1.4×10-9 ).

Answer 1

C₅H₅N + H₂O <-> C₅H₅NH⁺ + OH^-

$Kb = {\frac {[C_5H_5NH^+][OH^-]} {[C_5H_5N]}}=1.4*10^{-9}$

If x moles from 0.125 have dissociated then

$Kb = {\frac {[C_5H_5NH^+][OH^-]} {[C_5H_5N]}}={\frac {x^2} {0.125-x}}=1.4*10^{-9}$

0.125*1.4*10^-9 - 1.4*10^-9x = x²

x² + 1.4*10^-9x - 1.75*10^-10 = 0

Solving the equation and getting the positive root we get

x = 1.323*10^-5 = [OH^-]

pH = 14 - pOH

pOH = -lg[OH^-] = -lg(1.323*10^-5) = 4.88

pH = 14 - 4.88 = 9.12

pH = 9.12