Answer:
Option e.
Acid oxalate / oxalate
Explanation:
To determine the most efecttive buffer, we must look the pKas of the system.
If pKa value is similiar to buffer's pH, the variation of pH by the addition of a small amount of strong acid or base is minimal. It would be a good buffer.
a. H₃PO₄ / H₂PO₄⁻ Ka 1 = 6.9×10⁻³
pKa = - log Ka → - log 6.9×10⁻³ = 2.16
b. H₂CO₃ / HCO₃⁻ Ka1 = 4.8×10⁻⁷
pKa = - log Ka → - log 4.8×10⁻⁷ = 6.32
c. HCO₃⁻ / CO₃⁻² Ka2 = 4.8×10⁻¹¹
pKa = - log Ka → - log 4.8×10⁻¹¹ = 10.32
d. H₂S / HS⁻ Ka1 = 8.9×10⁻⁸
pKa = - log Ka → - log 8.9×10⁻⁸ = 7.05
e. HC₂O₄⁻ / C₂O₄⁻² Ka2 = 5.1×10⁻⁵
pKa = - log Ka → - log 5.1×10⁻⁵ = 4.29
pKa which is so close to the ph's value is e, acid oxalate / oxalate
