In: Chemistry
You prepare a buffer solution from 10.0 mL of 0.100 M MOPS (3-morpholinopropane-1-sulfonic acid) and 10.0 mL of 0.085 M NaOH. Next, you add 1.00 mL of 1.71 × 10-5 M lidocaine to this mixture Denoting lidocaine as L, calculate the fraction of lidocaine present in the form LH .
moles of MOPS = 0.1 x 10 / 1000 = 0.001
moles of OH- = 10 x 0.085 / 1000 = 0.00085
MOPS + OH- -----------------------------> Salt + H2O
0.001 0.00085 0 0
1.5 x 10^-4 0 0.00085 --
pH = pKa + log [salt / acid ]
pH = 7.2 + log ( 0.00085 / 1.5 x 10^-4)
pH = 7.95
[H+] = 10^-7.95
= 1.12 x 10^-8 M
Ka = Kw / Kb = 10^-14 / 8.7 x 10^-7 = 1.15 x 10^-8
fraction of lidocanine = H+ / H+ + Ka
= 1.12 x 10^-8 / (1.12 x 10^-8) + (1.15 x 10^-8)
= 0.493
fraction of lidocaine = 0.493 ----------------> answer