$(a)$ The ratio of hydrogen and oxygen in water is $1:8$ by mass. Find out their ratio by number of atoms in one molecule of water.
(Atomic mass: $H=1 \ u, O=16 \ u)$
$(b)$ Write the formulae of the following compounds:
$(i)$ Ammonium sulphate
$(ii)$ Magnesium chloride
(Given: Ammonium $= NH_{4}^{+}$,Sulphate $= SO_{4}^{2-}$,Magnesium $= Mg^{2+}$,Chloride $= Cl^{-}$)

(D) To find the ratio by number of atoms,we divide the mass ratio by the atomic mass of each element:

Element	Ratio by mass / Atomic mass
$H$	$1 / 1 = 1$
$O$	$8 / 16 = 0.5$

Dividing by the smallest value $(0.5)$:
$H = 1 / 0.5 = 2$
$O = 0.5 / 0.5 = 1$
Thus,the ratio of the number of atoms of $H$ to $O$ is $2:1$.
$(b)$ $(i)$ Ammonium sulphate: The valency of $NH_{4}^{+}$ is $+1$ and $SO_{4}^{2-}$ is $-2$. By criss-cross method,the formula is $(NH_{4})_{2}SO_{4}$.
$(ii)$ Magnesium chloride: The valency of $Mg^{2+}$ is $+2$ and $Cl^{-}$ is $-1$. By criss-cross method,the formula is $MgCl_{2}$.