Calculate the number of moles of magnesium present in a magnesium ribbon weighing $12 \, g$. The molar atomic mass of magnesium is $24 \, g \, mol^{-1}$. (in $, mol$)

Calculate the number of moles in $90 \text{ g}$ of water. (in $\text{ moles}$)

Calcium chloride when dissolved in water dissociates into its ions according to the following equation:
$CaCl_{2}(aq) \rightarrow Ca^{2+}(aq) + 2Cl^{-}(aq)$
Calculate the number of ions obtained from $CaCl_{2}$ when $222 \, g$ of it is dissolved in water.

$(i)$ $(a)$ What do you understand by $1 \text{ amu}$ or $1 \text{ u}$?
$(b)$ How is it linked with relative atomic mass?
$(ii)$ How do we know the presence of atoms if they do not exist independently for most of the elements?

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

$A$ gas jar contains $1.7 \, g$ of ammonia gas. Calculate the following:
$(i)$ Molar mass of ammonia.
$(ii)$ How many moles of ammonia are present in the gas jar?
$(iii)$ How many molecules of ammonia are present in the sample?