Calculate the mass of $20$ water molecules.

Calculate the number of particles in each of the following:
$(a)$ $48 \,g$ of $Mg$
$(b)$ $8 \,g$ of $O_2$
$(c)$ $0.1$ mole of carbon (Atomic mass $Mg = 24 \,u, O = 16 \,u, C = 12 \,u$,Avogadro constant $= 6.022 \times 10^{23}$)

Write the chemical formulae for the following compounds and find the ratio by mass of the combining elements in each one of them.
$(N=14 \ u, H=1 \ u, C=12 \ u, O=16 \ u, S=32 \ u)$
$(a)$ Methane
$(b)$ Carbon dioxide
$(c)$ Hydrogen sulphide

In photosynthesis,$6$ molecules of carbon dioxide combine with an equal number of water molecules through a complex series of reactions to give a molecule of glucose having a molecular formula $C_6H_{12}O_6$. How many grams of water would be required to produce $18\, g$ of glucose? Compute the volume of water (in $cm^3$) so consumed assuming the density of water to be $1\, g\, cm^{-3}$.

The chemical formula of the oxide of an element $X$ is $X_2O_5$. Write the chemical formula of its chloride.

$(a)$ Define atomicity.
$(b)$ Give an example for an element which is monoatomic and a polyatomic molecule.