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Question

In aqueous solution the ionization constants for carbonic acid are
K1=4.2×107 and K2=4.8×1011
Select the correct statement for a saturated 0.04 M solution of the carbonic acid

A
The concentration of CO23 0.034 M.
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B
The concentration of CO23 is greater than that of HCO3
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C
The concentrataion of H+ and HCO3 are approximately equal.
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D
The concentration of H+ is double that of CO23
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Solution

The correct option is C The concentrataion of H+ and HCO3 are approximately equal.
AH2CO3(aq.)H+(aq.)+HCO3(aq.) K1=4.2×107BHCO3(aq.)H+(aq.)+CO23(aq.) K2=4.8×1011As,K2<<K1. So, all major part of [H+] comes from dissociation of H2CO3.
[H+]total[H+]A and from first equilibrium.
[H+]A[HCO3][H+]total
[CO23] is negligible compared to
[HCO3] or [H+]total
Theory :
Ka for weak acids
For the following reaction : HA(aq) + H2O(l)H3O+(aq)+A(aq)
Ka=[H3O+][A][HA]
Kb for weak bases
For the following reaction : MOH(aq) + H2O(l)M+(aq)+OH(aq)
Kb=[M+][OH][MOH]
Note : More the values of Ka or Kb more is the strength of acid or base

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