Question

In aqueous solution the ionization constants for carbonic acid are K1 = 4.2 × 10–7 and K2 = 4.8 × 10–11

Select the correct statement for a saturated 0.034 M solution of the carbonic acid.

• A. The concentration of CO32– is 0.034 M.
• B. The concentration of CO32– is greater than that of HCO3–.
• C. The concentration of H+ and HCO3– are approximately equal.
• D. The concentration of H+ is double that of CO32–.

Answer 1

Answer: (C)

The concentration of H+ and HCO3– are approximately equal.

Answer 2

H2CO3 $\rightleftharpoons$ H+ + HCO3– K1 = 4.2 × 10–7

HCO3– $\rightleftharpoons$ H+ + CO32– K2 = 4.8 × 10–11 K1 >> K2

\ [H+] = [HCO3–]K2 = $\frac { \left[ \mathrm { H } ^ { + } \right] \left[ \mathrm { CO } _ { 3 } ^ { 2 - } \right] } { \left[ \mathrm { HCO } _ { 3 } ^ { - } \right] }$

but [H+] = [HCO3–]

[CO32–] = K2 = 4.8 × 10–11