$A$ gold sample contains $90\%$ of gold and the rest copper. How many atoms of gold are present in one gram of this sample of gold?

Calculate the number of aluminium ions present in $0.051 \, g$ of aluminium oxide.
[Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of $Al = 27 \, u$.]

One mole of $H_{2}O$ means:
$(i)$ $18 \text{ g}$ of $H_{2}O$
$(ii)$ $6.022 \times 10^{23}$ molecules of $H_{2}O$
$(iii)$ $180 \text{ g}$ of $H_{2}O$
$(iv)$ $1.8 \text{ g}$ of $H_{2}O$

What is the mass of :
$(a)$ $0.2$ mole of oxygen atoms?
$(b)$ $0.5$ mole of water molecules?

Which of the following has the maximum number of atoms?

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.