Identify the reagent '$$A$$' used in the following conversion.

Ethyl bromide $$\stackrel{\mathrm{A}}{\longrightarrow}$$ Ethylpropanoate

What is the concentration of $$\left[\mathrm{H}_3 \mathrm{O}^{+}\right]$$ ion in $$\mathrm{mol} ~\mathrm{L}^{-1}$$ of $$0.001 ~\mathrm{M}$$ acetic acid $$(\alpha=0.134)$$ ?

Which group elements from following are called as chalcogens?

What is the number of electrons around sulfur in $$\mathrm{H}_2 \mathrm{SO}_4$$ molecule?