Differentiate between:
$(i)$ Molar mass and Avogadro's constant.
$(ii)$ Molecular mass and formula unit mass.

Calculate the number of particles in each of the following:
$(a)$ $48 \,g$ of $Mg$
$(b)$ $8 \,g$ of $O_2$
$(c)$ $0.1$ mole of carbon (Atomic mass $Mg = 24 \,u, O = 16 \,u, C = 12 \,u$,Avogadro constant $= 6.022 \times 10^{23}$)

Carbon-containing molecules present in all life forms include:

$(a)$ $A$ 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?
$(b)$ Write one word for the following:
$(i)$ In a balanced chemical equation,the sum of the masses of reactants and products remains unchanged.
$(ii)$ $A$ group of atoms carrying a fixed charge on them.
$(c)$ Write chemical formulae of the following compounds:
$(i)$ Sodium phosphate
$(ii)$ Ammonium carbonate

$(a)$ What are molecules? Using examples, differentiate between the molecules of an element and a compound.
$(b)$ Derive the chemical formulae of the molecules formed between the following ions:
$(i)$ $Al^{3+}$ and $SO_{4}^{2-}$
$(ii)$ $Ba^{2+}$ and $NO_{3}^{-}$

$(a)$ In this crossword puzzle (Fig),names of $11$ elements are hidden. Symbols of these are given below. Complete the puzzle.
$1. Cl$
$2. H$
$3. Ar$
$4. O$
$5. Xe$
$6. N$
$7. He$
$8. F$
$9. Kr$
$10. Rn$
$11. Ne$
$(b)$ Identify the total number of inert gases,their names,and symbols from this crossword puzzle.