1.

Consider a solution consisting of the following two buffer systems. {:(H_(2)CO_(3)hArrHCO_(3)^(-)+H^(+),pKa=6.4),(H_(2)PO_(4)^(-)hArrHPO_(4)^(2-)+H^(+),pKa=7.2):} At pH = 6.4, which one of the following is true for the concentration of acid and conjugate base present

Answer»

`[H_(2)CO_(3)]gt[HCO_(3)^(-)]&[H_(2)PO_(4)^(-)]gt[HPO_(4)^(2-)]`
`[H_(2)CO_(3)]=[HCO_(3)^(-)]&[HPO_(4)^(2-)]gt[H_(2)PO_(4)^(-)]`
`[HCO_(3)^(-)]gt [H_(2)CO_(3)[&[HPO_(4)^(2-)]gt[H_(2)PO_(4)^(-)]`
`[H_(2)CO_(3)]=[HCO_(3)^(-)]&[H_(2)PO_(4)^(-)]gt[HPO_(4)^(2-)]`

SOLUTION :`"At pH"=6.4`
As in CASE of
`pH=pKa` it will be only when
`"[weak acid]"="[conjugate base]"`
In case of `H_(2)PO_(4)^(-)|HPO_(4)^(2-)`
`pH=pKa+log.([HPO_(4)^(2-)])/([H_(2)PO_(4)^(-)])`
`6.4=7.2+log.([HPO_(4)^(2-)])/([H_(2)PO_(4)^(-)])`
`log.([HPO_(4)^(2-)])/([H_(2)PO_(4)^(-)])=-ve""([HPO_(4)^(2-)])/([H_(2)PO_(4)^(-)])LT1""rArr[HPO_(4)^(2-)]LT[H_(2)PO_(4)^(-)]`


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