Which of the following is the most effective buffer system for a pH value of 4.45? a) H3PO4/H2PO4 (Kal for H3PO4 is 6.9 times 10-3) b) H_2CO_3/HCO_3^- (K_a1 for H_2CO_3 is 4.8 times 10^-7) c) HCO_3^-/CO_3^2- (K_a2 for H2CO_3 is 4.8 times 10^-11) d) H_2S/HS^- (K_a1 for H_2S is 8.9 times 10^-8) e) HC_2O_4^-/C_2O_4^2- (K_a2 for H_2C_2O_4 is 5.1 times 10^-5)

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