In which of the following does phosphorus have an oxidation state of $+3$?

The oxidation number of sulphur in $H_2S$ is

Dichromate ion is treated with base,the oxidation number of $Cr$ in the product formed is .... .

$A$ compound contains atoms of three elements $A$,$B$ and $C$. If the oxidation number of $A$ is $+2$,$B$ is $+5$ and that of $C$ is $-2$,the possible formula of the compound is:

What are the oxidation states of phosphorus in the following:
$(i)$ $H_{3}PO_{3}$
$(ii)$ $PCl_{3}$
$(iii)$ $Ca_{3}P_{2}$
$(iv)$ $Na_{3}PO_{4}$
$(v)$ $POF_{3}$

The equivalent weight of $MnSO_4$ is half of its molecular weight when it is converted into .....