Hello, I can answer this for you. You have to first look at the total molecular weight of phosphorus, which is H3PO4. Since H=1, P=98, and O=16 you simply add them together. H3PO4=(1*3)+(98)+(16*4)=165 Out of this weight Phosphorus accounts for 98/165, or 59.4% But since the solution is only 85% concentrated you have to multiply the two together, or .85*.549=51%
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Thanks, XXXXX XXXXX good sense to me.