Question

A buffer solution contains both a weak acid (HA) and its conjugate base (A-) so buffers do not have large changes in pH after addition of a small amount of strong acid or strong base.

What is the pH of 500.0 mL of a buffer composed of acetic acid (0.300 moles) and sodium acetate (0.200 moles) before and after addition of solid sodium hydroxide (5.84 g)? Assume no change in volume with addition of NaOH. pK_a = 4.756 for acetic acid.

Answer 1

pH = 5.108

Explanation

Number of moles of NaOH added = 5.84g/40g/mol = 0.146mol

Initial moles of CH3COOH = 0.300mol

Initial moles of CH3COO^- = 0.200mol

NaOH as a base react with weak acid CH3COOH

CH3COOH + OH^- -------> CH3COO^-   + H2O

0.146moles of NaOH react with 0.146moles of CH3COOH to producr 0.146moles of CH3COO^-

After addition of NaOH

moles of CH3COOH = 0.300mol - 0.146mol = 0.154mol

moles of CH3COO^- = 0.200mol + 0.146mol = 0.346mol

[CH3COOH] = (0.154mol/500ml) ×1000ml = 0.308M

[CH3COO^-] = (0.346mol/500ml)×1000ml = 0.692M

pKa of CH3COOH = 4.756

Henderson-Hasselbalch equation which give pH of a buffer solution is

pH = pKa + log([A^-]/[HA])

pH = 4.756 + log(0.692M/0.308M)

pH = 4.756 + 0.352

pH = 5.108