`K_(a)` for the reaction, `Fe^(3+)(aq.)+H_(2)O(l)hArr Fe(OH)^(2+)(aq.)+H_(3)O^(+)(aq.)` is `6.5xx10^(-3)` What is the max. pH value which could be used so that at least `80%` of the total iron (III) in a dilute solution exists as `Fe^(3+)`?

Options

A. `2.0`
B. `~2.4`
C. `~2.8`
D. `~1.6`

Answer

D

Explanation

`{:(Fe^(3+)(aq)+,H_(2)O(l)hArr,Fe(OH)^(2+)(aq)+,H_(3)O^(o+)(aq),),(1,-,0,0,),(C(1-alpha),,Calpha,Calpha,),((1-alpha=(80)/(100)=0.8,alpha=0.2),,,,):}`
`K_(a) = (Calpha)/(1-alpha)`
`6.5 xx 10^(-3) = (Cxx(0.2)^(2))/(0.8) :. C = 13.0 xx 10^(-2)`
`[H_(3)O^(o+)] = Calpha = 13.0 xx 10^(-2) xx 0.2 = 26 xx 10^(-3)`
`pH=- log (26 xx 10^(-3)) =- 1.415 +3 = 1.585 ~~ 1.6`.