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Question

Given an aqueous solution of 0.034 M carbonic acid, the ionization constants for carbonic acid are K1=4.2×107 and K2=4.8×1011
Select the correct statement from the following:
  1. The concentration of CO23 is greater than that of HCO3.
  2. The concentration of CO23 is 0.034 M.
  3. The concentration of H+ and HCO3 are approximately equal.
  4. The concentration of H+ is double that of CO23.


Solution

The correct option is C The concentration of H+ and HCO3 are approximately equal.
we can also express dissociation of  H2CO3 simpler way as:
Step 1 :
           H2CO3(aq)H+(aq)+HCO3(aq)
at t=0     C                      0                 0     
at t=teq  C(1x)           x+y         xy
Step 2 :
           HCO3(aq)H+(aq)+CO23(aq)
at t=teq  (xy)      x+y            yx=[H+] and [HCO3] and y=[CO23]
In many cases with Ka1>>Ka2
x>>y
xyx  and  x+yx
Note:- If C>>x so CxC then C(1α1)C
Ka1(x)(x)(Cx) where Cx=[H2CO3] and x=[H+] and [HCO3]
Ka2(y)(x)(x)y 

Therefore as , [H+]=x+yx=[HCO3]. So , Option c is correct.
 

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