Question

Find the concentration of `H^(o+), HCO_(3)^(Θ)`, and `CO_(3)^(-2)` in a `0.01M` solution of carbonic acid if the `pH` of solution is `4.18`.
`K_(1) = 4.45 xx 10^(-7),K_(2) = 4.69 xx 10^(-11)`

Answer 1

`pH =-log[H^(+)]`
`4.18=-log[H^(+)]`
`[H^(+)]=6.61xx10^(-5)`
`H_(2)CO_(3)hArrH^(+)+HCO_(3)^(-)`
`K_(a_(1))=([H^(+)][HCO_(3)^(-)])/([H_(2)CO_(3)])`
`or 4.45xx10^(-7)=(6.61xx10^(-5)xx[HCO_(3)^(-)])/(0.01)`
`therefore[HCO_(3)^(-)]=6.73xx10^(-5)M`
Again `HCO_(3)^(-)hArrH^(+)+CO_(3)^(-2)`
`K_(a_(2))=([H^(+)][CO_(3)^(-2)])/([HCO_(3)^(-)])`
`K_(a_(2))=4.69xx10^(-11)`
`=(6.61xx10^(-5)[CO_(3)^(-2)])/(6.73xx10^(-5))`
`[CO_(3)^(-2)]=4.8xx10^(-11)M`