Answer to Question #147529 in General Chemistry for gia

Question

Novocaine, used as a local anesthetic by dentists, is a weak base (Kb = 8.91 x 10^-6).

What is the ratio of the concentration of the base to that of its acid in the blood plasma (pH = 7.40) of a patient?

Answer 1

The pH is known, and K_b is known, so calculate [^-OH], then rearrange the K_b equation to get the ratio of the base to acid forms: [Novocaine]/[Novocaine•H⁺].

pH = 7.40, so pOH = (14 - 7.40) = 6.60 Therefore, [^-OH] = 10^-6.60 = 1.58 × 10^-7 M

Rearrange:

Kb = [−OH][Novocaine⋅H+]/

[Novocaine]

to give:

     [Novocaine]/      

                                      =   [⁻OH]      

[Novocaine⋅H⁺] K_b

So:

[Novocaine]/[Novocaine•H⁺] = 1.58 × 10^-7 / 8.91 × 10^–6

therefore, the ratio is = 1.8 × 10^–2