$A$ sample of vitamin $C$ is known to contain $2.58 \times 10^{24}$ oxygen atoms. How many moles of oxygen atoms are present in the sample?

What is the mass of one mole of oxygen atoms (in $g$)?

Give two examples to show that the law of conservation of mass applies to physical changes also.

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 ...............

$(a)$ The ratio of hydrogen and oxygen in water is $1:8$ by mass. Find out their ratio by number of atoms in one molecule of water.
(Atomic mass: $H=1 \ u, O=16 \ u)$
$(b)$ Write the formulae of the following compounds:
$(i)$ Ammonium sulphate
$(ii)$ Magnesium chloride
(Given: Ammonium $= NH_{4}^{+}$,Sulphate $= SO_{4}^{2-}$,Magnesium $= Mg^{2+}$,Chloride $= Cl^{-}$)

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