The first step in the synthesis of phosphoric acid is the reaction of phosphorus and oxygen to form diphosphorus pentoxide.
[tex] P_{4}(l) + 5 O_{2}(g)--->2 P_{2}O_{5} (g) [/tex]
The second step in the synthesis of phosphoric acid is the reaction of diphosphorus with water to form phosphoric acid.
[tex] P_{2}O_{5}(g) + 3H_{2}O (l) --> 2H_{3}PO_{4}(aq) [/tex]
The net chemical equation representing the formation of phosphoric acid from phosphorus, water and oxygen will be,
[tex] Step -1 : P_{4}(l) + 5O_{2}(g) ---> 2P_{2}O_{5}(g) [/tex]
[tex] Step 2: P_{2}O_{5}(g) + 3 H_{2}O(l)-->2 H_{3}PO_{4}(aq)[/tex]
Equation in step 2 must be multiplied by 2 in order to cancel diphosphorus pentoxide.
[tex] P_{4}(l) + 5O_{2}(g)---> 2P_{2}O_{5} [/tex]
[tex] 2 [P_{2}O_{5} (g) + 3 H_{2}O(l) --> 2 H_{3}PO_{4} (aq) ] [/tex]
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Net: [tex] P_{4}(l) + 5 O_{2}(g) + 6 H_{2}O(l) ---> 4H_{3}PO_{4}(aq) [/tex]
