Which of the following species have $18$ electrons? $Ca^{2+}, K^{+}, Na, Cl^{-}, Ar$. Also,state the reason why the chemical properties of all isotopes of an element are similar.

Compute the difference in masses of $10^{3} \, \text{mol}$ each of magnesium atoms and magnesium ions. (Mass of an electron $= 9.1 \times 10^{-31} \, \text{kg}$)

The chemical symbol for nitrogen gas is

$A$ sample of ethane $(C_2H_6)$ gas has the same mass as $1.5 \times 10^{20}$ molecules of methane $(CH_4)$. How many $C_2H_6$ molecules does the sample of gas contain?

State the number of hydrogen atoms in $1\,g$ of hydrogen.

$(a)$ Calculate the number of moles in $112 \, g$ of iron.
$(b)$ Calculate the mass of $0.5$ mole of sugar $(C_{12}H_{22}O_{11})$.
$(c)$ Calculate the number of atoms in $8 \, g$ of oxygen $(O_2)$ molecules.
(Atomic mass of $Fe = 56 \, u, C = 12 \, u, H = 1 \, u, O = 16 \, u, N_A = 6.022 \times 10^{23} \, mol^{-1}$)