what is the oxidation number of arsenic in chromium(III) arsenate?

Question

Pressure treated lumber contains a compound with the molecular formula CrAsO₄. Given that this compound contains a Cr(III) cation, what is the oxidation number of arsenic in chromium(III) arsenate?

a) As (I)

b)As (III)

c)As(IV)

d)As(V)

*I am between b and d, but more going with option b. Since I think we also need to think about the oxygen charge, no?

Answer 1

Yes, you need to consider the oxidation number of oxygen.

CrAsO4

Cr = 3+

O = 2- and 4 of them = 8-

Therefore, As must be 5+ or As(V), so option d would be the correct answer.

If it were option b where As had a +3 then you'd have +3 for Cr, +3 for As (total of +6) and -8 for oxygen, giving a net charge of -2 which is not present. The oxidation numbers must add to zero for a neutral compound.