Compute the difference in masses of one mole each of aluminium atoms and one mole of its ions. (Mass of an electron is $9.1 \times 10^{-28} \,g$). Which one is heavier (in $,g$)?

Write the molecular formulae of all the compounds that can be formed by the combination of the following ions: $Cu^{2+}, Na^{+}, Fe^{3+}, Cl^{-}, SO_{4}^{2-}, PO_{4}^{3-}$.

$A$ chlorine atom is electrically neutral,but a chloride ion is a charged ion. Explain.

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 $H_{2}O$

Complete the following crossword puzzle by using the names of the chemical elements based on the data provided in the table.

Across	Down
$2$. The element used by Rutherford during his $\alpha$-scattering experiment.	$1$. $A$ white lustrous metal used for making ornaments and which tends to get tarnished black in the presence of moist air.
$3$. An element which forms rust on exposure to moist air.	$4$. Both brass and bronze are alloys of this element.
$5$. $A$ very reactive non-metal stored under water.	$6$. The metal which exists in the liquid state at room temperature.
$7$. Zinc metal when treated with dilute hydrochloric acid produces a gas of this element which when tested with a burning splinter produces a pop sound.	$8$. An element with the symbol $Pb$.

Fill in the blanks:
$(a)$ In a chemical reaction,the sum of the masses of the reactants and products remains unchanged. This is called ...............
$(b)$ $A$ group of atoms carrying a fixed charge on them is called ...............
$(c)$ The formula unit mass of $Ca_3(PO_4)_2$ is ...............
$(d)$ The formula of sodium carbonate is ............... and that of ammonium sulphate is ...............