An element `A` in a compound `ABD` has oxidation number `A^(n-)`. It is oxidised by `Cr_(2)O_(7)^(2-)` in acid medium. In the experiment `1.68xx10^(-3)` moles of `K_(2)Cr_(2)O_(7)` were used for `3.26xx10^(-3)` moles of `ABD`. The new oxidation number of `A` after oxidation is:

Explanation

Redox change involved are:
`A^(n-) to A^(a+) +(a+n)e^(-)`
`6e^(-)+Cr_(2)^(6+) to 2Cr^(3+)`
meq. Of `A^(n-)="meq. Of "Cr_(2)O_(7)^(2-)`
or `3.26 xx 10^(-3) xx (a+n)=1.68 xx 10^(-3) xx 6`
`a+3 =3 rArr a=3-n`