Question

If the pk values for phosphoric acid are 2.15, 6.82 and 12.38, _____ would predominate at ph 5 while _____ would predominate at ph 10.

Answer 1

Phosphoric acid
`H_(3)PO_(4)` have three protons which is ionized in different
`P^(H)`. Thats why it is called triprotic acid. The protonisation of phosphoric acid in different
`P^(H)` can be shown as-
`H_(3)PO_(4)`→
`H^(+)+H_(2)PO_(4)^(-1)`. This is the first ionization. The second and third ionizations are shown as-
`H_(2)PO_(4)^(-1)`→
`H^(+)`+
`HPO_(4)^(2-)` and
`HPO_(4)^(2-)`→
`H^(+)` + P
`O_(4)^(3-)`

`p^(H)`=5 is near to first equivalence point where
`p^(H)`=
`(1)/(2)` (
`p_(k)^(1)` +
`p_(k)^(2)`=
`(1)/(2)`(2.15+6.82)=4.6. At this
`p^(H)`,
`H_(2)PO_(4)^(-1)` will be present.

At
`p^(H)`=10, which is near to second equivalence point
`p^(H)=(1)/(2) (p_(k)^(2) + p_(k)^(3))`=
`(1)/(2)`(6.82+12.38)=9.7. At
`p^(H)`=10,
`HPO_(4)^(2-)` will be present.