Question

Preparation of the acetic acid-sodium acetate buffer:

Calculate the theoretical pH of this buffer solution.

3.504 grams of NaC₂H₃O₂*3H₂O with 8.8 mL of 3.0 M acetic acid and 55.6 mL of distilled water.

Experimental pH of buffer solution: 4.8

Answer 1

Theoretical pH of acidic buffer solution
No. of moles of weak acid (acetic acid) = 8.8 x 10-3 x 3 = 2.64 x 10-4 = 0.0264 moles
No. of moles of conjugate base salt (sodiam acetate trihydrate) = 3.5/136.08= 0.0257 moles
the dissociation constant (Ka ) for acetic acid is 1.8 x 10-5
total volume = 8.8 x 10-3 + 55.6 x 10-3 = 6.44 x 10-2 L
Concentartion of acetic acid [HA-] in buffer = 0.0264 / 6.44 x 10-2 = 0.4099 moles/L
Concentartion of conjugate base [A-] in buffer = 0.0257/ 6.44 x 10-2 = 0.3991 moles/L
pKa = -log Ka
pKa = - log (1.75 x10-5)
pKa= 4.76
pH= pKa + log [ A-]/[HA]
pH= 4.76 + log 0.3991/0.4099
pH=4.76 + (-0.0116)
therefore theoretical pH= 4.75