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In aqueous solution the ionisation constants for carbonic acid are

K_{1}=4.2\times 10^{-7}\; \; and\; \; K_{2}=4.8\times 10^{-11}

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

  • Option 1)

    The concentration of H+ is double that of  CO_{3}^{2-}

  • Option 2)

    The concentration of CO_{3}^{2-} is 0.034 M.

  • Option 3)

    The concentration of CO_{3}^{2-} is greater than that of HCO_{3}^{-}.

  • Option 4)

    The concentration of H+  and HCO_{3}^{-} are approximately equal.

 

Answers (1)

best_answer

As we have learned

Ionization constant of weak acids -

Consider 

HX(aq)+H_{2}O(l)\rightleftharpoons H_{3}O^{+}(aq)+\bar{X}(aq)
 

K_{a}=dissociation\:or\:ionization\:constant

- wherein

K_{a}=\frac{C^{2}\:\alpha ^{2}}{C(1-\alpha )}

K_{a}=\frac{C\alpha ^{2}}{1-\alpha }
 

C=initial\:concentration\:of\:undissociated\:acid

\alpha =extent\:upto\:which\:HX\:is\:ionized\:into\:ions.

 

 H_2CO_3 \rightleftharpoons H^+ + HCO^-\; \; \; \; \; \; \; k_1 = 4.2 \times 10 ^{-7}

HCO_3^+ \rightleftharpoons H^+ + CO_{3}^{2-}\; \; \; \; \; \; \; k_2 = 4.8 \times 10 ^{-11}

K_1 >>K_2

\therefore [ H^+] = [HCO_{3}^{-}] \Rightarrow k_2 = \frac{[H^+][10_{3}^{2-}]}{[HCO_{3}^{-}]}

So , [ CO_{3}^{2-}]= k_2 = 4.8 \times 10 ^{-11}

Secdond dissasociation constant is muchg smaller than the first one .Just a small fraction of total HCO_{3}^{-}    formed will under go the second stage of ionisation 

 

 

 

 

 


Option 1)

The concentration of H+ is double that of  CO_{3}^{2-}

Option 2)

The concentration of CO_{3}^{2-} is 0.034 M.

Option 3)

The concentration of CO_{3}^{2-} is greater than that of HCO_{3}^{-}.

Option 4)

The concentration of H+  and HCO_{3}^{-} are approximately equal.

Posted by

SudhirSol

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