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

Let the oxidation state of $P$ be $x$.
$(i)$ $H_{3}PO_{3}$: $3(+1) + x + 3(-2) = 0 \implies 3 + x - 6 = 0 \implies x = +3$.
$(ii)$ $PCl_{3}$: $x + 3(-1) = 0 \implies x - 3 = 0 \implies x = +3$.
$(iii)$ $Ca_{3}P_{2}$: $3(+2) + 2(x) = 0 \implies 6 + 2x = 0 \implies 2x = -6 \implies x = -3$.
$(iv)$ $Na_{3}PO_{4}$: $3(+1) + x + 4(-2) = 0 \implies 3 + x - 8 = 0 \implies x - 5 = 0 \implies x = +5$.
$(v)$ $POF_{3}$: $x + (-2) + 3(-1) = 0 \implies x - 5 = 0 \implies x = +5$.