What is the mass of :
$(a)$ $0.2$ mole of oxygen atoms?
$(b)$ $0.5$ mole of water molecules?

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.
$\text{Sodium carbonate} + \text{Acetic acid} \to \text{Sodium acetate} + \text{Carbon dioxide} + \text{Water}$

What is the mass of:
$(a)$ $1$ mole of nitrogen atoms?
$(b)$ $4$ moles of aluminium atoms (Atomic mass of aluminium $= 27$)?
$(c)$ $10$ moles of sodium sulphite $(Na_2SO_3)$?

What is meant by the term chemical formula?

Write the chemical formulae of the following:
$(i)$ Magnesium chloride
$(ii)$ Calcium oxide
$(iii)$ Copper nitrate
$(iv)$ Aluminium chloride
$(v)$ Calcium carbonate.