`H_(3)A` is a weak triprotic acid `(K_(a1)=10^(-5),K_(a2)=10^(-9),K_(a3)=10^(-13)`
What is the value of pX of 0.1 M `H_(3)A` (aq.) solution ? Where pX=-log X and X=`[[A^(3-)]]/[[HA^(2-)]]`

Options

A. `7`
B. `8`
C. `9`
D. `10`

Answer

D

Explanation

`alpha` is negligible w.r.t `1`
`[H^(+)]` mainly from first step
`[H^(+)] = sqrt(K_(a) xx C) = sqrt(10^(-5) xx 0.1) = 10^(-3)`
`:. K_(a_(3)) = ([H^(+)][A^(3-)])/([HA^(2-)]) = 10^(-13) = (10^(-3)[A^(3-)])/([HA^(2-)])`
`X = ([A^(3-)])/([HA^(2-)]) = 10^(-10)` ,
`:. pX = 10`