Equilibrium

Equilibrium

Question

In aqueous solution the ionization constants for carbonic acid are
K1 = 4.2 x 10-7 and K2 = 4.8 x 10-11.
Select the correct statement for a saturated 0.034 M solution of the carbonic acid.

  • The concentration of CO32- is 0.034 M

  • The concentration of CO32- is greater than that of HCO3-

  • The concentration of H+ and HCO3- are approximately equal

  • The concentration of H+ is double that of CO32-

Answer

C.

The concentration of H+ and HCO3- are approximately equal

straight H subscript 2 CO subscript 3 space rightwards harpoon over leftwards harpoon with space on top space straight H to the power of plus space plus space HCO subscript 3 superscript minus space semicolon space straight K subscript 1 space equals space 4.2 space straight x space 10 to the power of negative 7 end exponent
HCO subscript 3 superscript minus space rightwards harpoon over leftwards harpoon with space on top space straight H to the power of plus space plus space CO subscript 3 superscript 2 minus end superscript space semicolon space straight K subscript 2 space equals space 4.8 space straight x space 10 to the power of negative 11 end exponent
straight K subscript 1 space greater than greater than space straight K subscript 2
therefore space left square bracket straight H to the power of plus right square bracket space equals space left square bracket HCO subscript 3 superscript minus right square bracket semicolon space straight K subscript 2 space equals space fraction numerator left square bracket straight H to the power of plus right square bracket left square bracket CO subscript 3 superscript 2 minus end superscript right square bracket over denominator left square bracket HCO subscript 3 superscript minus right square bracket end fraction
So comma space left square bracket CO subscript 3 superscript 2 minus end superscript right square bracket space equals space straight K subscript 2 space equals space 4.8 space straight x space 10 to the power of negative 11 end exponent
Second dissociation constant is much smaller than the first one. Just a small fraction of total HCO3- formed will undergo the second stage of ionization. Hence in a saturated solution.

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