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-)` is greater than that of `HCO_(3)^(-)`
C. The concentration of `H^(+)` and `HCO_(3)^(-)` are approximately equal.
D. The concentration of `H^(+)` is double that of `CO_(3)^(2-)`

Answer 1

`(1)` and `(4)` are incorrect since `H_(2)CO_(3)` is not a strong acid .
`{:(HCO_(3)hArr H^(+)+HCO_(3)^(-)," ",K_(1)=4.2xx10^(-3)),(HCO_(3)^(-)hArr H^(+)+CO_(3)^(2-)," ",K_(2)=4.8xx10^(-11)):}`
`:. K_(1) gtgt `, so ` H^(+)` produced from `II^(nd)` ionisation can be neglected as compared to those produced in `1^(st)` ionisation.
`:.[H^(+)]=[HCO_(3)^(-)]` (from `1^(st)` ionisation)
Also ,`[CO_(3)^(2-)] ltlt [HCO_(3)^(-)]`( `:.II ^(nd)` ionistion will occur upto negligible extent).