b. P--->P2Ọ--->H3PO4-->NaH2PO4-->Na3PO4

b.

 $4P+5O_2\stackrel{t^o}{\to }2P_2{O}_5$

$P_2{O}_5+3H_{2}O\to 2H_{3}P{O}_4$

$H_{3}P{O}_4+NaOH\to Na{H}_{2}P{O}_4+H_{2}O$

$Na{H}_{2}P{O}_4+NaOH\to N{a}_{2}HP{O}_4+H_{2}O$

$N{a}_{2}HP{O}_4+NaOH\to N{a}_{3}P{O}_4+H_{2}O$