You prepare a buffer solution from 10.0 mL of 0.100 M MOPS (3-morpholinopropane-

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 0.081 M NaOH. Next, you add 1.00 mL of 5.97 × 10-4 M lidocaine to this mixture Denoting lidocaine as L, calculate the fraction of lidocaine present in the form LH .

u prepare a buffer solution from 10.0 mL of 0.100 M MOPS (3- OH orpholinopropane-1-sulfonic acid) and 10.0 mL of 0.081 M NaOH. Next, you add 1.00 mL of 5.97 × 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 x10-8 Number CH3 CH3 Lidocaine Kb 8.7 x10-7

Explanation / Answer

Using Hendersen-Hasselbalck equation,

pH = pKa + log([base]/[acid])

From MOPS

[RSO3H] = (0.1 M x 10 ml - 0.081 M x 10 ml)/20 ml = 0.0095 M

[RSO3-] = 0.081 M x 10 ml/20 ml = 0.0405 M

pH = 7.20 + log(0.0405/0.0095)

     = 7.83

When 1 ml of 5.97 x 10^-7 M lidocaine is added at pH 7.83

pOH = 14 - 7.83 = 6.17

[OH-] = 6.76 x 10^-7 M

Kb = [LH][OH-]/[L]

[LH]/[L] = 8.7 x 10^-7/6.76 x 10^-7 = 1.29 is the fraction of LH/L

(or (1.29/2.29) x 100 = 56.33%)

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