Question

Suppose there is 1.00 L of an aqueous buffer containing 60.0 mmol of acetic acid (pK = 4.76) and 40.0 mmol of acetate. Calculate the pH of this buffer. pH = What volume of 5.00 M NaOH would be required to increase the pH to 4.93? volume:

Answer 1

1)
use:
pH = pKa + log {[conjugate base]/[acid]}
= 4.76+ log {0.0400/0.060}
= 4.584

Answer: 4.58

2)
Let volume of NaOH be V L
mol of NaOH added = 5*V mol

Before adding NaOH

Before Reaction:
mol of CH3COO- = 0.04 mol

mol of CH3COOH = 0.06 mol

5*V NaOH will react with 5*V of CH3COOH to form extra 5*V of base
After adding NaOH
mol of CH3COOH = 0.06-5*V mol
mol of CH3COO- = 0.04+5*V mol
use:
pH = pKa + log {[conjugate base]/[acid]}
4.93 = 4.76+log {[CH3COO-]/[CH3COOH]}
log {[CH3COO-]/[CH3COOH]} = 0.17
[CH3COO-]/[CH3COOH] = 1.479


So,
(0.04+5*V)/(0.06-5*V) = 1.4791
0.04+5*V = 0.0887 - 7.3955*V
(5+7.3955)*V = 0.0887-0.04
V = 0.00393 L
V = 3.93 mL
Answer: 3.93 mL