How is the silver halide obtained from an organic compound distinguished?

Why is nitric acid $(HNO_3)$ added to sodium extract before adding silver nitrate $(AgNO_3)$ for testing halogens?

Given below are two statements :
Statement $I :$ In Lassaigne's test,the covalent organic molecules are transformed into ionic compounds.
Statement $II :$ The sodium fusion extract of an organic compound having $N$ and $S$ gives prussian blue colour with $FeSO_4$ and $FeCl_3$.
In the light of the above statements,choose the correct answer from the options given below.

The formulae of ammonium phosphomolybdate $(X)$ and the compound $(Y)$ responsible for Prussian blue colour are:

When sodium fusion extract of an organic compound is boiled with iron$(II)$ sulphate solution followed by addition of concentrated $H_2SO_4$,it gives a Prussian blue colour. This confirms the presence of which element?

Name the methods used for determining the molecular mass of organic compounds.