Question

If two compounds have the same empirical formula but different molecular formula, they must have:
(A) Different percentage composition
(B) Different molecular weight
(C) Same vapour density
(D) None of these

Answer 1

Hint: An empirical formula is used to represent the simplest whole-number ratio of various atoms which are present in a compound whereas the molecular formula shows the exact composition of different types of atoms present in a molecule of a compound. That means an empirical formula can be the same for more than one compound but the molecular formula is unique for each compound except the case of isomers.

Complete Step by Step Solution:
The percentage composition of a compound is the percentage of mass of each atom contributing in a molecule. In case when a compound’s formula is unknown, we can measure the mass of each of its constituent elements and is often considered as the first step in the process of determining the formula experimentally.

The percentage composition of the compound is used for determining the empirical formula of the compound whose molecular formula is unknown. The percentage composition and empirical formula can be the same for more than two compounds. Therefore, if two compounds have the same empirical formula but different molecular formula, they must have different molecular weight.
For example, consider the following two compounds:
Formaldehyde: $C{H_2}O$
Acetic acid: ${C_2}{H_4}{O_2}$

The given compounds have the same percentage compositions because the ratio of elements in the compound i.e., the empirical formula is the same but their molecular formulas are different and thus have different molecular weight.
So, option (B) is the correct answer.

Note: It is important to note that the vapour density of a compound can be defined as the ratio of mass of certain volume of a gas to the mass of an identical volume of hydrogen and the relationship between molecular weight and vapour density can be defined as Molecular weight $ = \;2\; \times $ vapour density. Thus, if two compounds have the same empirical formula but different molecular formula, they must have different vapour density.