Question

In aqueous solution the ionization constants for carbonic acid are:
`K_(1)=4.2xx10^(-7)and K_(2)=4.8xx10^(-11)`
Select the correct statement for a saturated `0.034M` solution of the carbonic acid.
A. The concentration of `CO_(3)^(2-)` is `0.034M`
B. The concentration of `CO_(3)^(2-)` si greater than that of `HCO_(3)^(-)`
C. The concentration of `H^(+)` and `HCO_(3)^(-)` are approximately equal
D. The concentratation of `H^(+)` is double that of `CO_(3)^(2-)`

Answer 1

Correct Answer - C
(i): `H_(2)CO_(3)hArr H^(+)+HCO_(3)^(-), K_(1)=4.2xx10^(-7)`
(ii) : `HCO_(3)^(-)hArr H^(+)+CO_(3)^(2-), K_(2)= 4.8xx10^(-11)`
Because, `K_(2)ltltK_(1)`, major part of `[H^(+)]` comes from dissociation of `H_(2)CO_(3)`
`[H^(+)]_("total")~~[H^(+)]_(A)`
and from 1 equilibrium
`[H^(+)]_(A)~~[HCO_(3)^(-)]~~[H^(+)]_("total")`
`[CO_(3)^(2-)]` is negligible compared to `[HCO_(3)^(-)]` or `[H^(+)]_(total)` .