You have 5mL of 1M CH3COOH and want to titrate using 1M NaOH. How much NaOH do you need to reach neutrality?

Answer 1

Neutrality suggests you want a final pH of 7.0, which for a weak acid, acetic acid (CH3COOH) titrated with a strong base (NaOH) will NOT be at the equivalence point. The reason is that during the titration, a buffer is formed containing the weak acid and the conjugate base (CH3COO-). In order to calculate the "amount" of NaOH needed, we can use the Henderson Hasselbalch equation: pH = pKa + log [conjugate base]/[acid]. First, we look up the pKa for acetic acid. It is 4.76.

CH₃COOH + OH^- ===> CH₃COO^- + H₂O

7.0 = 4.76 + log [conj.base]/[acid]

log [CH₃COO^-]/[CH₃COOH] = 7.0 - 4.76 = 2.24

[CH₃COO^-]/[CH₃COOH] = 174 meaning that the ratio of conjugate base : acid should be 174 : 1

CH₃COOH + OH^- ===> CH₃COO^- + H₂O

0.005............x................0.........................Initial

-x................-x...............+x.......................Change

0.005-x........0................x........................Equilibrium

[CH₃COO-]/[CH₃COOH] = 174

(x/0.005-x) = 174

x = 0.00497 moles NaOH need to be added

(x L)(1 mol/L) = 0.00497 mol and x = 0.00497 L = 4.97 mls of NaOH need to be added

NOTE: Addition of 4.97 mls of 1 M NaOH to 5.0 mls of 1 M weak acid results in neutrality at a point slightly below that of the equivalence point