State whether the following statement is true or false:
Iron shows valencies of $Fe^{2+}$ and $Fe^{3+}$.

What is the difference between a cation and an anion?

Write the molecular formulae for the following compounds:
$(a)$ Copper $(II)$ bromide
$(b)$ Aluminium $(III)$ nitrate
$(c)$ Calcium $(II)$ phosphate
$(d)$ Iron $(III)$ sulphide
$(e)$ Mercury $(II)$ chloride
$(f)$ Magnesium $(II)$ acetate

$(a)$ Calculate the percentage of oxygen in $H_{2}O_{2}$.
$(b)$ Calculate the number of moles in $56 \, g$ of nitrogen gas $(N_{2})$.
$(c)$ Calculate the mass of $0.50$ moles of $Ca$ atoms.
$(d)$ Find out the number of particles in:
$(i)$ $5$ moles of methane gas $(CH_{4})$
$(ii)$ $92 \, g$ of sodium $(Na)$
(Atomic masses: $H=1, O=16, N=14, Ca=40, Na=23$)

State whether the following statement is true or false:
$A$ mole of any gas always occupies $22.4$ litres of volume at $0^{\circ} C$ and $1$ atmospheric pressure,which is known as standard temperature and pressure $(STP)$.

Derive the molecular formula for Ammonium carbonate.