What is the oxidation number of P in H3PO3?

'3'

H3PO3

To calculate oxidation no. First sepatate out the ion.

In this case PO3^(3-).

Using oxygen as tha standard at '-2'.

Then the oxygen moiety is '3 X -2 = -6'

Creating a little sum

P + -6 = -3 (The charge on the ion) .

Add '6' t

NB 'P' is in the same group as 'N' . Elements in this Group have variable oxid'n numbers.

Hence

P -6 + 6 = -3 +6

P = +3 The oxidation number of 'P'.