Define the atomic mass unit.

How many atoms are present in a $(i)$ $H_2S$ molecule and $(ii)$ $PO_4^{3-}$ ion?

In a reaction,$5.3 \, g$ of sodium carbonate reacted with $6 \, g$ of acetic acid. The products were $2.2 \, g$ of carbon dioxide,$0.9 \, g$ of water,and $8.2 \, g$ of sodium acetate. Show that these observations are in agreement with the law of conservation of mass.
$\text{Sodium carbonate} + \text{Acetic acid} \to \text{Sodium acetate} + \text{Carbon dioxide} + \text{Water}$

Calculate the molecular masses of $H_2$,$O_2$ and $Cl_2$.

Calculate the molar mass of the following substances.
$(a)$ Phosphorus molecule,$P_4$ (Atomic mass of phosphorus $= 31$)
$(b)$ Hydrochloric acid,$HCl$
$(c)$ Nitric acid,$HNO_3$

Calculate the molecular masses of $CO_2$,$CH_4$ and $C_2H_6$.