What is meant by the following?
$(i)$ Molar mass
$(ii)$ Valency

Which of the following correctly represents $360 \, g$ of water?
$(i)$ $2$ moles of $H_2O$
$(ii)$ $20$ moles of water
$(iii)$ $6.022 \times 10^{23}$ molecules of water
$(iv)$ $1.2044 \times 10^{25}$ molecules of water

Give three differences between an atom and a molecule.

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

$(a)$ What do the following formulae stand for:
$(i)$ $2O$
$(ii)$ $O_{2}$
$(iii)$ $O_{3}$
$(iv)$ $H_{2}O$
$(b)$ Give the chemical formulae of the following compounds:
$(i)$ Potassium carbonate
$(ii)$ Calcium chloride
$(c)$ Calculate the formula unit mass of $Al_{2}(SO_{4})_{3}$.
(Given atomic mass of $Al = 27 \, u, S = 32 \, u, O = 16 \, u$)

The mass of one steel screw is $4.11 \, g$. Find the mass of one mole of these steel screws. Compare this value with the mass of the Earth $(5.98 \times 10^{24} \, kg)$. Which one of the two is heavier and by how many times?