You prepare a buffer solution from 10.0 mL of 0.100 M MOPS (3-morpholinopropane-1-sulfonic acid) and 10.0 mL of 0.080 M NaOH. Next, you add 1.00 mL of 3.83 × 10-4 M lidocaine to this mixture Denoting lidocaine as L, calculate the fraction of lidocaine present in the form LH . MOPS KA=6.3*10^-8 Lidocaine Kb=8.7*10^-7
10.0 mL of 0.100 M MOPS is 0.01L x 0.1 = 0.001 moles
10.0 mL of 0.080 M NaOH is 0.01 L x 0.08 = 0.0008 moles in 20 mL is 0.04M
After reaction with NaOH acid form is 0.001-0.0008 = 0.0002 in 20 mL is 0.01M
pKa = -log Ka = - log 6.3 x 10-8 = 7.2
So
pH = pKa + log base/acid
pH = 7.2 + log 0.04/0.01
pH = 7.8
lidocaine as L Kb = 8.7 x 10-7 Ka = 1 x 10-14/ 8.7 x 10-7 = 1.15 x 10-8; pKa = - log Ka = 7.94
pH = pKa + log base/acid
7.8 = 7.94 + log base/acid
log base/acid = -0.14
base/acid = 10-0.14 = 0.725
If base= 0.725 acid form = 1
So LH form is 1/1.725 = 0.58 fraction is LH form
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