The hydrogen is throwing my a bit off because I don't know what its oxidation number is, thus preventing me from figuring out what the oxidation number of phosphorus.
3 Answers
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H has +1 charge.
O has -2 charge.
So....
2(+1) + P + 4(-2) = -1
2 - 8 + P= -1
P=+5
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+5
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H is +1
PO4 is a polyatomic ion and always is -3
so it should be H3PO4