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Oxidation can be calculated as:
 H₃PO₃ 
oxidation state of hydrogen is +1
oxidation state of oxygen is -2

calculation for P oxidation state is :
1 x3+ P +(-2) x3 =0
3+P+(-6)=0
3+P=6
P=6-3 
=3
here oxidation state is +3

(ii) PCl₃
Oxidation state of chlorine is -3
thus
P+(-3) =0
P = 3
here oxidation state is +3

(iii) Ca₃P₂

oxdation state of calcium is +2
 2 x3 +2P =0
6+ 2P =
2P=-6
P= -6/2 =-3

oxidation state is -3

(iv)Na₃PO₄ 
here oxidation state of sodium is +1
oxidation state of oxygen is -2
1 x3 +P +(-2) x4=0
3 +P +(-8) =0
3+P =8
P=8-3 =5
oxidation state is +5

(v) POF₃
oxidation state fulorine is (-1)
oxidation state is (-2)
P+(-2) +(-1) x3 =0
P= 5
oxidation state is +5

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