(A) One molecule of the substance

It is given that the valency of iron in iron sulphate is $+3$ and we know that for sulphate $({SO _4}^{2-})$, it is $-2$. Therefore, the formula is $Fe _2(SO _4) _3$.

Formula of first oxide = $M _3O _4$ 
let mass of the metal be = x
percentage of metal in $\displaystyle M _3O _4 = \frac{3x}{ 3x+64}\times100$
but % age = (100-27.6) = 72.4 %
so, $\displaystyle \frac{3x}{ 3x+64}\times100$
or x = 56.
in 2nd oxide,
oxygen = 30%....so metal = 70%
so, ratio :--
M : O
70/56 : 30/16
1.25 : 1.875
2 : 3
so, 2nd oxide =$ M _2O _3$

Potassium permangnate is a strong oxidizing agent and an inorganic chemical compound which is used in titrations. It has formula ${ KMnO } _{ 4}\ consisting\ of\ { K }^{ + },{ MnO } _{ 4 }^{ - }\quad ions.$ In this compound manganese is in $+7$ oxidation state.

Correct chemical formula's
(i) $Zn _3(PO _4) _2$ 
(ii) $Fe _2S _3$
(iii) $AlN$ 
(iv) $Ca(HCO _3) _2$ 
(v) $K _2SO _4$.

Correct chemical formula's
(i) $ZnSO _4$ 
(ii) $Fe _2S _3$
(iii) $Mg _3N _2$ 
(iv) $Ca(HO) _2$ 
(v) $KCl$.

The incorrect formula's are; $AZ _{3}$, $B _{2}X$, $C _{3}Y _{2}$

 $Ca^{2+}$ ion and $H _{2}PO _{4}^{-1}$ ion combine to form $Ca(H _{2}PO _{4}) _{2}$

$MHPO _4$ is derived from the acid $H _{3}PO _{4}$ . Thus the metal $M$ replaced two hydrogen atoms of hydrogen phosphate. So, its valency is $2.$ Hence the formula of the metal chloride is $MCl _{2}$ . (Because the valency of$ M$ is $2$ and the valency of$ Cl$ is $1$

Option(C) is incorrect. It can be $CaSO _{4}$ and not  $CaHSO _{4}$.

Molecular formulas describe the exact number and type of atoms in a single molecule of a compound. The constituent elements are represented by their chemical symbols, and the number of atoms of each element present in each molecule is shown as a subscript following that element's symbol. The molecular formula expresses information about the proportions of atoms that constitute a particular chemical compound, using a single line of chemical element symbols and numbers. Sometimes it also includes other symbols, such as parentheses, dashes, brackets, and plus (+) and minus (–) signs.

 A chemical formula that shows the total number and kinds of atoms in a molecule, but not their structural arrangement or the molecular formula is a short form representation of a compound.

When naming a binary compound, the number of atoms of that element in the compound determines the prefix for an element in a compound.
For example, while naming  $N _2O _3 $, the prefix di is used for $N$ which indicates $2 N$ atoms and the prefix tri is used for $O$ which indicates $3 O$ atoms. Hence, the name dintirogen trioxide.

The salt of an oxyacid of a metal M contains one sulphur and four oxygen atoms. The oxyacid of the salt is $H _{2}SO _{4}$. One sulphur and four oxygen atoms indicate sulphate ion $(SO _4^{2-})$. Hence, the salt is metal sulphate and the oxyacid is sulphuric acid.

Trivial name of $Na _2CO _3\longrightarrow$ Soda Ash

The general formula of one molecule of a metallic halide can be-

Carbon has a valency of$ +4$ and chlorine has a valency equal to $-1. $Compound is Carbon tetrachloride $CCl _4$.