Question

Chemical formula of few compounds are given:
$(i)Z{n_3}{(P{O_4})_2}(ii)SFe(iii)AI{N_3}(iv)Ca{(HC{O_3})_2}(v){K_2}S{O_4}.$
The wrong chemical formula are:
A) (ii) and (iii)
B) (i),(iv) and (v)
C) (i) and (iv)
D) (i), (ii) and (iv)

Answer 1

Hint: A molecular formula consists of the chemical symbols for the constituent elements followed by numeric subscripts describing the number of atoms of each element present in the molecule. The empirical formula represents the simplest whole-integer ratio of atoms in a compound.

Complete answer:
Write down the symbols of the ions, which combine to form a molecule of the compound, side by side. While writing the formula of a compound containing a metal and a non-metal, the symbol of the metal is written first followed by that of the non-metal.
Write the valency of each ion.
Interchange the valencies of the ions and write as the subscript. Write them close together and ignore them to obtain the formula.
The polyatomic ions must be written in brackets, before the subscript is written. In case the number of polyatomic ions is one, the bracket is not required.
Let’s check if all the formulas are correct or not:
$(i)\;Z{n_3}{(P{O_4})_2}$
The valence electron in zinc is $2$ and charge on phosphorus is $ + 3$. By using a cross multiplication method. Formula is $Z{n_3}{(P{O_4})_2}$
So, this formula is correct.
$(ii)\;SFe$
The valence electron in $8$ and it has a $6$ electron in its outermost shell and charge on the sulphur ion is $ - 2$. So, $SFe$ is wrong .
The correct chemical formula is $F{e_2}{S_6}$
$(iii)\;AI{N_3}$
The chemical formula for $AL{N_3}$ is ${C_9}{H_8}{O_4}$. This is also wrong.
$(iv\;)Ca{(HC{O_3})_2}$
The charge on the calcium is $ + 2$ and charge on $HC{O_3}$ is $ - 1$. so, by using cross multiplication method the formula is $Ca{(HC{O_3})_2}$
So, this formula is correct
$V)\;{K_2}S{O_4}$
The charge on potassium is $ + 1$ and charge on $S{O_4}$ is $ - 2$ .so, by using cross multiplication method the formula is ${K_2}S{O_4}$
So, this is also correct.
The correct answer is A) (ii) and (iii)

Note:
Remember molecular formula and structural formula are two different things. A molecular formula uses chemical symbols and subscripts to indicate the exact numbers of different atoms in a molecule or compound. A structural formula indicates the bonding arrangement of the atoms in the molecule.