Question

Given an aqueous solution of 0.034 M carbonic acid, the ionization constants for carbonic acid are K1=4.2×10−7 and K2=4.8×10−11 Select the correct statement from the following:The concentration of CO2−3 is greater than that of HCO−3.The concentration of CO2−3 is 0.034 M.The concentration of H+ and HCO−3 are approximately equal.The concentration of H+ is double that of CO2−3.

Solution

The correct option is C The concentration of H+ and HCO−3 are approximately equal. we can also express dissociation of  H2CO3 simpler way as: Step 1 :            H2CO3(aq)⇌H+(aq)+HCO−3(aq) at t=0     C                      0                 0      at t=teq  C(1−x)           x+y         x−y Step 2 :            HCO−3(aq)⇌H+(aq)+CO2−3(aq) at t=teq  (x−y)      x+y            yx=[H+] and [HCO−3] and y=[CO2−3] In many cases with Ka1>>Ka2,  x>>y ∴x−y≈x  and  x+y≈x Note:- If C>>x so C−x≈C then C(1−α1)≈C Ka1≈(x)(x)(C−x) where C−x=[H2CO3] and x=[H+] and [HCO−3] Ka2≈(y)(x)(x)≈y  Therefore as , [H+]=x+y≈x=[HCO−3]. So , Option c is correct.

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