Question

Empirical formula of compound is $C{H_2}O$ . If the molecular weight is 180, then the molecular formula of the compound is?
A) ${C_6}{H_{12}}{O_6}$
B) ${C_5}{H_{10}}{O_5}$
C) ${C_3}{H_6}{O_3}$
D) ${C_4}{H_5}{O_4}$

Answer 1

Molecular formula tells which atoms are present in molecules and how much. Empirical formula gives the simplest integer ratio of atoms in a molecule.

Complete answer:
Let us first look at the definitions of the terms required in the question
Definition of Molecular Formula:
It is the chemical formula of a molecule indicating all atoms individually and their amount or no. in which they are present in the molecule. For example, the molecular formula of water is ${H_2}O$ which shows that there are a total 2 atoms of hydrogen ( H ) and one atom of oxygen ( O ).
Definition of Empirical Formula:
It is a chemical formula of a molecule indicating the fraction by which all atoms are present in a molecule. For Example molecular formula of glucose is ${C_6}{H_{12}}{O_6}$ and its empirical formula will be $C{H_2}O$ as fraction of carbon hydrogen and oxygen will be 6:12:6 which is equal to 1:2:1 which is the simplest integral ratio possible for glucose.
Steps for finding Molecular Formula from given empirical formula and molecular mass:
First of all divide molar mass of compound by the mass of compound’s empirical formula.
Then you will find integer or a whole no. very close to integer. Round off the value to make it integer.
After that multiply this above calculated no. with subscripts in the empirical formula so the chemical formula you will find will be the molecular formula of the compound.
In the given question molecular weight of compound = 180
And molecular mass of empirical formula ($C{H_2}O$) will be= 12+2 $\times$ 1+16 = 30
Therefore ratio of their masses will be = 180 : 30= 6
Now multiplying this no. with subscripts of the $C{H_2}O$.
Therefore the molecular formula of $C{H_2}O$ will become ${C_6}{H_{12}}{O_6}$ .

So the correct option is (A).

Note: Empirical formula is also called a stoichiometric formula. It is the simplest formula which gives the lowest whole number ratio between the number of atoms of different elements present in the substance.