Explanation

(b)
Detection of phosphorus in the organic compound can be done by its conversion into phosphate. The ammonium molybdate is used to identify phosphate ion
$Na_{3}PO_{4} + 3HNO_{3} \rightarrow H_{3}PO_{4}\ 3NaNO_{3}$
$H_{3}PO_{4} + 12\left( NH_{4} \right)_{2}MoO_{4} + 21HNO_{3}\overset{\mathrm{\Delta}}{\rightarrow}$
ammonium modybdate
$\left( NH_{4} \right)_{3}PO_{4}.12MoO_{3} + 21NH_{4}NO_{3} + 12H_{2}O$
yellow ppt.