Calculate the mass of $20$ water molecules.

$A$ change in the physical state can be brought about:

$(a)$ 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 $NO_{3}^{-}$

Nitrogen and hydrogen combine in the ratio $14:3$ by mass to form an ammonia molecule. Find the formula of the ammonia molecule by calculating the molar ratio. (Given atomic mass of $N = 14 \ u, H = 1 \ u$)

As per the law of definite proportion,carbon and oxygen combine in the ratio of $3:8$. Compute the mass of oxygen gas that would be required to react completely with $15\,g$ of carbon.

Calculate the number of atoms in:
$(i)$ $0.02$ moles of carbon
$(ii)$ $30$ grams of carbon
$(iii)$ $6$ moles of aluminium ions.