Which of the following is the most effective buffer system for a pH value of 4.4

Question

Which of the following is the most effective buffer system for a pH value of 4.45?

H2CO3/HCO3–     (Ka1 for H2CO3 is 4.3 × 10–7.)

HCO3–/CO32–     (Ka2 for H2CO3 is 4.8 × 10–11.)

H2S/HS–     (Ka1 for H2S is 8.9 × 10–8.)

HC2O4–/C2O42–     (Ka2 for H2C2O4 is 5.1 × 10–5.)

H3PO4/H2PO4–     (Ka1 for H3PO4 is 6.9 × 10–3.)

H2CO3/HCO3–     (Ka1 for H2CO3 is 4.3 × 10–7.)

HCO3–/CO32–     (Ka2 for H2CO3 is 4.8 × 10–11.)

H2S/HS–     (Ka1 for H2S is 8.9 × 10–8.)

HC2O4–/C2O42–     (Ka2 for H2C2O4 is 5.1 × 10–5.)

H3PO4/H2PO4–     (Ka1 for H3PO4 is 6.9 × 10–3.)

Explanation / Answer

For most effective buffer system

pH value is close to pKa value.

So option C is answer.

Since pKa of bicarbonate anion = -log(5.1 × 10^-5)

= 5 - log(5.1) = 5 - 0.7 = 4.3

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