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?

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

Do $5$ moles of $CO_{2}$ and $5$ moles of $H_{2}O$ have the same mass?

Write the chemical names of the following compounds:
$(a)$ $K_{2}SO_{4}$
$(b)$ $Mg_{3}(PO_{4})_{2}$
$(c)$ $NH_{4}Cl$
$(d)$ $ZnS$
$(e)$ $Na_{3}N$
$(f)$ $AgBr$

$(a)$ State the law of conservation of mass.
$(b)$ What mass of silver nitrate will react with $5.85 \, g$ of sodium chloride to produce $14.35 \, g$ of silver chloride and $8.5 \, g$ of sodium nitrate,if the law of conservation of mass is true?

Compute the number of ions present in $5.85 \, g$ of sodium chloride.