In aqueous solution, almost all the carbon dioxide is present as

Options

A. `CO_(2)(aq)`
B. `H_(2)CO_(3)`
C. `HCO_(3)^(-)`
D. `CO_(3)^(2-)`

Answer

A

Explanation

When `CO_(2)` dissolves in water, only some of the molecules react with water to form corbonic acid, `H_(2)CO_(3)`, which has recently been isolated at low temperature and in the absence of water. In the presence of water, dissociation to carbon dioxide and water occurs rapidly.
Carbonic acid is an extremely weak diprotic (dibasic) acid, as can be seen from the `pK_(a)` values corresponding to each of the ionization steps:
`CO_(2)(g)+H_(2)O(l)hArrH_(2)CO_(3)(aq.)`
`H_(2)CO_(3)(aq.)+H_(2)O(l) hArr H_(3)O^(+)(aq.)+HCO_(3)^(-)(aq.),pK_(a1) = 6.37`
`HCO_(3)^(-)(aq.)+H_(2)O(l) hArr H_(3)O^(+)(aq.)+CO_(3)^(2-)(aq.),pK_(a2)=10.33`
Thus, a solution of `CO_(2)` in water is actually an equilibrium mixture of `CO_(2)`, `H_(2)CO_(3)^(-)`, and `CO_(3)^(2-)` . `CO_(2)` is only slightly hydrated to `H_(2)CO_(3)`, and the solution contains few carbonates or bicarbonate ions., Most of the dissolved `CO_(2)` remains loosely hydrated to give `CO_(2) (aq)`.