Compounds that should not be used as primary standards in titrimetric analysis are :

A. $\mathrm{Na}_2 \mathrm{Cr}_2 \mathrm{O}_7$

B. Oxalic acid

C. NaOH

D. $\mathrm{FeSO}_4 \cdot 6 \mathrm{H}_2 \mathrm{O}$

E. Sodium tetraborate

Choose the most appropriate answer from the options given below:

Choose the correct tests with respective observations.

(A) $\mathrm{CuSO}_4$ (acidified with acetic acid) $+\mathrm{K}_4\left[\mathrm{Fe}(\mathrm{CN})_6\right] \rightarrow$ Chocolate brown precipitate.

(B) $\mathrm{FeCl}_3+\mathrm{K}_4\left[\mathrm{Fe}(\mathrm{CN})_6\right] \rightarrow$ Prussian blue precipitate.

(C) $\mathrm{ZnCl}_2+\mathrm{K}_4\left[\mathrm{Fe}(\mathrm{CN})_6\right]$, neutralised with $\mathrm{NH}_4 \mathrm{OH} \rightarrow$ White or bluish white precipitate.

(D) $\mathrm{MgCl}_2+\mathrm{K}_4\left[\mathrm{Fe}(\mathrm{CN})_6\right] \rightarrow$ Blue precipitate.

(E) $\mathrm{BaCl}_2+\mathrm{K}_4\left[\mathrm{Fe}(\mathrm{CN})_6\right]$, neutralised with $\mathrm{NaOH} \rightarrow$ White precipitate.

Choose the correct answer from the options given below :

Identify the inorganic sulphides that are yellow in colour :

(A) $(NH_4)_2S$

(B) $PbS$

(C) $CuS$

(D) $As_2S_3$

(E) $As_2S_5$

Choose the correct answer from the options given below :