Cinnabar $(HgS)$ is a prominent ore of mercury. How many grams of mercury are present in $225 \,g$ of pure $HgS$ (in $,g$)? Molar mass of $Hg$ and $S$ are $200.6 \,g \,mol^{-1}$ and $32 \,g \,mol^{-1}$ respectively.

Which of the following postulates of Dalton's atomic theory is challenged by the next generation of scientists?

$(a)$ 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 $Fe = 56 \, u, C = 12 \, u, H = 1 \, u, O = 16 \, u, N_A = 6.022 \times 10^{23} \, mol^{-1}$)

What are ionic and molecular compounds? Give examples.

$(i)$ Name the body which approves the nomenclature of elements and compounds.
$(ii)$ The symbol of sodium is written as $Na$ and not as $S$. Give reason.
$(iii)$ Name one element which forms diatomic and one which forms tetraatomic molecules.

Which of the following species is electrically neutral and why?
$K^{+}, Cl^{-}, S^{2-}, P^{3-}, K$