Which of the following represents $1 \, u$?

Derive the molecular formula for Copper $(II)$ Bromide.

Calculate the number of molecules in $0.5$ mole of water.

Calculate the mass of:
$(a)$ $1$ atom of carbon
$(b)$ $0.02$ mole of sodium
$(c)$ $1$ molecule of water.

$(i)$ Name the body which approves the nomenclature of elements and compounds.
$(ii)$ The symbol of sodium is written as $Na$ and not as $S$. Give reason.
$(iii)$ Name one element which forms diatomic and one which forms tetraatomic molecules.

Fill in the blanks:
$(a)$ In a chemical reaction,the sum of the masses of the reactants and products remains unchanged. This is called ...............
$(b)$ $A$ group of atoms carrying a fixed charge on them is called ...............
$(c)$ The formula unit mass of $Ca_3(PO_4)_2$ is ...............
$(d)$ The formula of sodium carbonate is ............... and that of ammonium sulphate is ...............