Mohr's salt is

Rearrange the following ($I$ to $IV$) in the order of increasing masses and choose the correct answer (Atomic mass: $N=14, O=16, Cu=63$).
$I.$ $1$ molecule of oxygen
$II.$ $1$ atom of nitrogen
$III.$ $1 \times 10^{-10} \ g$ molecular weight of oxygen
$IV.$ $1 \times 10^{-10} \ g$ atomic weight of copper

Nitrogen atom has an atomic number of $7$ and oxygen has an atomic number of $8$. The total number of electrons in a nitrate ion $(NO_3^-)$ will be:

Iron sulphide is heated in air to form $A$, an oxide of sulphur. $A$ is dissolved in water to give an acid. The basicity of this acid is:

Write formulas for the following compounds:
$(1)$ Mercury $(II)$ chloride
$(2)$ Nickel $(II)$ sulphate
$(3)$ Tin $(IV)$ oxide
$(4)$ Thallium $(I)$ sulphate

How is concentrated $H_2SO_4$ diluted?