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

What is the percentage of lead in a lead pencil (in $\%$)?

Arrange the following in the order of increasing mass (atomic mass: $O = 16, Cu = 63, N = 14$)
$I.$ one atom of oxygen
$II.$ one atom of nitrogen
$III.$ $1 \times 10^{-10}$ mole of oxygen
$IV.$ $1 \times 10^{-10}$ mole of copper

The volume of $HCl$,containing $73 \ g \ L^{-1}$,required to completely neutralize $NaOH$ obtained by reacting $0.69 \ g$ of metallic sodium with water,is $........ mL$. (Nearest Integer) (Given: molar masses of $Na, Cl, O, H$ are $23, 35.5, 16$ and $1 \ g \ mol^{-1}$ respectively)

$16 \ g$ each of $H_2$,$He$,and $O_2$ are present in a container exerting $10 \ atm$ pressure at $T \ (K)$. The pressure in $atm$ exerted by $16 \ g$ each of $He$ and $O_2$ in the second container of same volume and temperature is

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: