Question

You prepare a buffer solution from 10.0 mL of 0.100 M MOPS (3-morpholinopropane-1-sulfonic acid) and 10.0 mL of 0.074 M NaOH. Next, you add 1.00 mL of 6.91 × 10-6 M lidocaine to this mixture

Denoting lidocaine as L, calculate the fraction of lidocaine present in the form LH .

Answer 1

Given that, Volume, V = 10.0 mL

No.of moles = 0.100 M

mmol = M x V

= 0.1 x 10

= 1.0 mmoles.

moles of NaOH = 10 mL x 0.074

= 0.74

Concentration of salt = 0.74 M

Concentration of acid = 1 - 0.74

= 0.26 M

We know, pKa = - log(Ka)

= -log(6.3 x 10^-8) . (Ka of MOPS is 6.3 x 10^-8)

= 7.20

Let us consider, pH = pKa+ log (0.74 / 0.26)

= 7.20 +0.45

= 7.65

Ka for Lidocaine = K_W / K_b

= 1 x 10^-14 / 8.7 x10^-7

= 1.14 x 10^-8

We know,   = [H⁺] / [(H⁺) +(Ka)]

[H⁺] = 10^-pH

= 10^-7.65

= 2.23 x 10^-8

   = [H⁺] / [(H⁺) +(Ka)]

= 2.23 x 10^-8/( 2.23 x 10^-8 + 1.14 x 10^-8)

     = 0.661