Use the Henderson-Hasselbalch equation to perform the following calculations.

Question

Use the Henderson-Hasselbalch equation to perform the following calculations.

The Ka of acetic acid is: 1.8 × 10–5. FW for sodium acetate, trihydrate (NaC2H3O2•3H2O) is 136.08 g/mol.

• Buffer A: Calculate the mass of solid sodium acetate required to mix with 50.0 mL of

0.1 M acetic acid to prepare a pH 4 buffer.

• Buffer B: Calculate the mass of solid sodium acetate required to mix with 50.0 mL of

1.0 M acetic acid to prepare a pH 4 buffer.

Answer 1

Henderson Hasselbalch equation: pH = pKa + log [conj.base] / [acid]

pKa = -log Ka = -log 1.8x10^-5 = 4.74

Buffer A:

4.00 = 4.74 + log [acetate]/[acetic acid]

log [acetate]/[acetic acid] = -0.74

[acetate]/[acetic acid] = 0.182

x/0.1 = 0.182

x = [acetate] = 0.0182 M

0.0182 mol/L x 0.050 L x 136.08 g / mol = 0.124 g sodium acetate, trihydrate

Since the molarity of the acetic acid is given to only 1 sig. fig. you might want to report the answer as 0.1 g

Buffer B:

Same as buffer A